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LABORATORY   MANUAL 


Direct  and  Alternating  Current 


LABORATORY  MANUAL 

DIRECT  AND  ALTERNATING  CURRENT 

Prepared  to  accompany  Timbie's  Elements  of  Electricity 


By 

CLARENCE  E.  CLEWELL 
\\ 

Sheffield  Scientific  School  of  Yale  University 


SECOND  EDITION 


NEW  HAVEN,  CONN. 
1913 


COPYRIGHT,  1913,   BY 
CLARENCE  E.  CLEWELL 


T  A-  i 'I ' 


Engineering 
Library 


PRESS  OF  THE  TIMES   PUBLISHING  COMPANY 
BETHLEHEM,  PA. 


PREFACE 

This  book  of  laboratory  directions  is  designed  for  the  use  of 
students  in  courses  other  than  that  of  Electrical  Engineering, 
who  take  a  brief  amount  of  work  in  the  fundamental  principles 
of  electricity. 

The  object  of  the  laboratory  work  in  such  cases  is  to  aid  in  the 
understanding  of  the  theoretical  and  practical  items  given  in  the 
recitation  and  lecture  room.  It  is  the  purpose,  therefore,  to  elimi- 
nate in  as  far  as  possible  all  features  in  the  actual  work  of  the 
laboratory  which  would  tend  to  lessen  or  detract  from  concen- 
tration upon  the  underlying  principles  involved  in  the  experi- 
ment in  hand. 

To  facilitate  the  work,  each  experiment  is  prefaced  where  possi- 
ble by  an  assignment  of  articles  in  the  text  book  which  bear  di- 
rectly or  indirectly  on  the  experiment  in  question.  Further,  in  a 
part  of  the  experiments,  diagrams  of  the  electrical  connections 
will  be  found,  together  with  simple  forms  which  may  be  followed 
in  the  recording  of  observations.  These  forms  are  somewhat 
more  complete  in  the  first  than  in  the  advanced  experiments  in 
order  to  serve  at  the  outset  as  a  guide  in  the  preparation  of  the 
data  sheets.  In  the  latter  experiments  this  general  scheme  of  rul- 
ing up  the  data  sheet  is  to  be  followed  by  each  group  of  men  in 
the  preparation  of  data  sheets  before  the  laboratory  exercise. 

After  teaching  with  a  text  book  which  was  too  advanced  for  the 
students  and  a  laboratory  manual  which  was  not  suited  to  the 
text  book,  the  students  nor  the  equipment,  it  was  thought  best  to 
write  a  Manual  which  should  be  adapted  to  the  specific  require- 
ments of  the  coming  year.  The  difficulties  encountered  by  the 
student  have  been  observed  and  the  aim  of  the  Manual  is  to  pre- 
sent the  subject  from  his  standpoint.  The  emphasis  placed  on 
such  practical  items  as  constant  potential  supply  mains  in  the 
wiring  diagrams,  and  the  practical  points  connected  with  direct 
and  alternating  current  generators  and  motors  and  power  trans- 


271653 


vi  PREFACE 

mission  in  the  text,  are  intended  to  familiarize  the  student  with 
the  principles  underlying  the  operation  of  standard  apparatus 
which  he  may  encounter  after  graduation. 

Appreciation  for  many  suggestions  and  helpful  advice  in  the 
make-up  of  this  book  is  due  Professor  Chas.  F.  Scott,  Sheffield 
Scientific  School  of  Yale  University. 


CLARENCE  E.  CLEWELL. 


NEW  HAVEN,  CONN., 
July,  1913. 


CONTENTS 


PA<JK 

Introduction    1 

DIRECT    CURRENT 
EXPERIMENT 

1.  Ohm's  Law 9 

2.  Measurement  of  Armature  and  of  Field  Resistance 13 

3.  A^oltage  and  Power  Losses  in  Transmission 15 

4.  Study  of  Measuring  Instruments 17 

5.  Study  of  Fuses  and  Circuit  Breakers 21 

6.  Study  of  Starting  Boxes  for  Motors 24 

7.  Study  of  Electric  Lamps 27 

8.  Building  up  of  the  Shunt  Generator 30 

9.  Electrical  Features  of  the  Shunt  Generator 34 

10.  Comparison  of  Shunt  and  Separate  Field  Excitation 37 

11.  Electrical  Features  of  the  Compound  Generator 40 

12.  Study  of  the  Storage  Battery 44 

13.  Study  of  Electric  Wiring. 46 

14.  Shunt  Motor  Speed  Features 48 

15.  Efficiency  of  the  Shunt  Motor  by  the  Brake  Method 51 

16.  Series  Motor  Speed  Features 53 

17.  Efficiency;   Stray  Power  Test;  Brake  Test 56 

18.  Static  Torque  Test  on  a  Motor 59 

19.  Shunt  Generators  in  Parallel 61 

20.  Compound  Generators  in  Parallel 65 

ALTERNATING    CURRENT 

21.  Resistance  and  Reactance  in  Series 71 

22.  Resistance  and  Reactance  in  Parallel 76 

23.  Study  of  Three-Phase  Circuits 79 

24.  Study  of  the  Transformer 83 

25.  Electrical  Features  of  the  Transformer  and  the  Transmission 

of  Power    86 

26.  Study  of  the  Induction  Motor 88 

27.  Electrical  Features  of  the  Induction  Motor 90 

28.  Study  of  the  Synchronous  Motor 93 

29.  Alternators  in  Parallel 96 

30.  Study  of  the  Mercury  Arc  Rectifier 98 


VII 


LABORATORY  MANUAL 


INTRODUCTION  TO  THE  LABORATORY  WORK. 

1.  General  Hints. — While  the  laboratory  work  is  intended 
primarily  as  an  aid  to  a  clear  understanding  of  the  text  book  and 
lecture  work,  to  be  successful,  it  must  be  performed  in  a  system- 
atic manner,  and  with  due  care  in  handling  electric  circuits, 
instruments  and  machinery. 

The  electric  current  is  somewhat  intangible  and  when  making 
connections  and  carrying  out  tests,  the  only  way  to  conduct  the 
work  intelligently  is  to  form  at  the  outset  definite  ideas  of  the 
laws  which  govern  the  flow  of  current  in  the  circuit  involved. 

A  most  important  item  to  observe  is  the  distinction  between 
electric  pressure  (electromotive  force)  expressed  in  volts,  and 
electric  current  expressed  in  amperes,  and  quantitative  ideas 
should  be  gained  early  in  the  work  regarding  the  usual  values 
of  these  units.  The  most  common  value  of  pressure  in  com- 
mercial lighting  is  approximately  110  volts.  The  normal  voltage 
is  different  in  different  plants ;  it  may  be  108,  or  110,  or  115,  etc. 
In  each  case  the  aim  is  to  maintain  the  normal  value  at  all  times. 
The  general  term  "110  volts"  is  used  to  mean  a  constant  po- 
tential circuit  of  the  110-volt  class.  An  ordinary  carbon  filament 
incandescent  lamp  consumes  one-half  an  ampere  with  a  pressure 
of  110  volts  across  its  terminals.  The  principal  circuits  used  in 
the  laboratory  tests  are  110  and  220  volts.  Electric  railway  cir- 
cuits are  ordinarily  550  volts,  and  five  110-volt  lamps  are  con- 
nected in  series  in  the  cars  for  lighting. 

In  connecting  up  a  set  of  apparatus  and  the  instruments  for  a 
test,  some  idea  should  be  had  of  what  each  component  part  of  the 
circuit  stands  for  in  its  relation  to  the  whole,  as  well  as  a  fairly 
definite  idea  of  what  will  happen  when  the  main  switch  is  closed. 
As  a  safeguard,  the  Instructor  should  always  be  asked  to  check 


2  LABORATORY  MANUAL 

over  the  connections  before  power  is  applied  through  the  closing 
of  the  main  switch. 

As  to  the  connections  themselves,  a  switch,  together  with  a  fuse 
in  each  side  of  the  line,  should  be  installed.  The  switch  should  be 
closed  and  opened  quickly  at  the  start  to  see  if  all  is  correct,  and 
after  the  switch  has  been  closed  and  the  work  started,  each  piece 
of  apparatus  should  be  watched  closely  for  the  first  few  minutes 
to  note  if  any  part  of  the  circuit  is  being  overheated.  Should  any 
trouble  exist,  open  the  switch  at  once  and  trace  the  connections 
to  find  the  error.  A  heavy  current  will  flow  through  a  low  re- 
sistance at  110  or  220  volts,  and  hence,  in  using  apparatus  having 
low  resistance  an  adequate  additional  resistance  must  always  be 
placed  in  the  circuit  before  closing  the  switch. 

2.  Material  Required  for  the  Work. — Each  man  is  to  have 
the  following  items : 

(a)  Paper  for  use  in  recording  data  with  carbon  paper  for 
duplicating  the  data  sheets. 

(b)  A  pad  of  ruled  loose  leaf  note  paper  on  which  the  report 
is  to  be  written,  together  with  suitable  report  covers. 

(c)  A  pad  of  cross  section  paper  the  same  size  as  the  loose  leaf 
paper  for  curve  plotting. 

It  is  suggested  that  students  provide  themselves  with  a  pair  of 
wrireman  's  pliers  and  a  screw  driver. 

3.  Instruments. — Electrical  measuring  instruments  are  deli- 
cate and  expensive  and  they  should  be  handled  with  the  same 
care  that  one  would  use  in  the  handling  of  a  high  grade  watch 
or  equivalent  piece  of  jewelled  apparatus.     Damage  to  instru- 
ments through  carelessness  is  chargeable  to  the  students  involved. 

Ammeters. — An  ammeter  is  a  low  resistance  instrument  and  is 
intended  for  the  measurement  of  the  electric  current  which  flows 
through  its  coil.  The  important  item  is  to  avoid  sending  through 
the  ammeter  a  current  which  is  larger  than  it  is  intended  to 
carry.  A  safeguard  is  a  short  circuiting  switch  as  shown  in  Fig. 
1.  When  this  switch  is  closed  the  current  flows  through  the 
switch  and  the  ammeter  is  thus  protected.  When  the  observa- 
tions are  to  be  made,  the  switch  is  opened  and  the  current  flows 
through  the  instrument.  This  precaution,  while  not  always 
necessary,  is  sufficiently  important  to  make  it  desirable  in  the 
early  experiments. 


INTRODUCTION 


Millivoltmeters  and  Shunts. — A  resistance  of  known  value  R 
may  be  connected  in  the  circuit  (the  shunt  in  Fig.  1),  and  a  low 
reading  voltmeter  known  as  the  millivoltmeter  ("thousandths  of 
a  volt"  meter)  may  be  connected  across  its  terminals  to  deter- 
mine the  current  flowing  through  the  resistance  R.  If  e  is  the 
reading  on  the  millivoltmeter  in  millivolts  (thousandths  of  a 
volt),  the  current  is  then  equal  to 


-i-  R   or 


E  (volts) 


1000  R 

These  instruments  may  be  calibrated  to  read  amperes  directly, 
or  what  is  more  common  in  the  laboratory,  the  shunt  is  stamped 


Supply  Mains  (110  Volts  D  C.) 

JL 

—  M        [  1— 

Mam  Switch 

m      m 


/ 

rt  Circuiting  / 
Switch  / 


To  the  Load 


Fig.  1. — Two  types  of  instrument  for  measuring  current:  (a)  am- 
meter, carrying  the  full  current;  (b)  shunt  with  millivoltmeter  for 
measuring  the  voltage  drop  in  the  shunt.  Note  the  short  circuiting 
switch  for  the  protection  of  the  instrument  in  each  case. 

with  the  current  which  flows  through  the  circuit  when  the  milli- 
voltmeter to  which  it  is  connected  indicates  its  full  scale  deflec- 
tion. Thus  a  millivoltmeter  whose  full  scale  deflection  is  100, 
when  used  with  a  shunt  stamped  10,  indicates  10  amperes  when 
the  millivoltmeter  registers  its  full  deflection,  that  is,  each  scale 
division  indicates  one-tenth  of  an  ampere.  All  shunts  in  the 
laboratory  are  numbered  according  to  the  instrument  with  which 
they  are  to  be  used,  and  in  all  cases  the  number  on  the  shunt  and 
on  the  instrument  must  correspond, 


4  LABORATORY  MANUAL 

Voltmeters. — A  voltmeter  is  a  high  resistance  instrument  and, 
hence,  is  actuated  by  a  relatively  small  current  flowing  through 
its  coil.  The  voltmeter  is  intended  for  use  across  a  circuit  and 
measures  the  pressure  E  effective  between  its  terminals.  If  the 
resistance  of  the  voltmeter  is  R,  and  it  is  connected  across  110- 
volt  mains,  the  current  flowing  through  the  instrument  is  110/R. 
Thus  the  voltmeter  is,  in  reality,  a  current-measuring  instrument 
on  a  small  scale,  but  its  divisions  are  calibrated  in  volts  and  the 
current  is  usually  a  very  small  fraction  of  an  ampere  or  a  negli- 
gible quantity  in  most  of  the  practical  experiments.  The  range 
of  a  voltmeter  may  be  increased  by  the  use  of  an  external  supple- 
mental resistance  termed  a  multiplying  coil,  although  in  many 
of  the  simpler  tests  this  is  unnecessary,  since  instruments  of  a 
suitable  range  are  available. 

4.  Use  of  Instruments. — Always  keep   an  instrument  away 
from  stray  magnetic  fields  like  those  in  the  neighborhood  of  some 
electromagnets  and  some  generators  and  motors.     Outside  mag- 
netism may  affect  the  accuracy  of  an  instrument  very  appre- 
ciably. 

Where  the  wires  from  an  instrument  cross  the  floor  and  there 
is  danger  of  it  being  pulled  off  the  table,  the  wires  should  be  an- 
chored to  a  table  leg  or  should  pass  through  holes  in  the  table 
provided  for  this  purpose. 

No  instrument  should  be  used  where  the  reading  to  be  taken 
is  less  than  one-third  of  its  full  scale  deflection,  since  the  accuracy 
of  an  instrument  is  greater  for  higher  than  for  lower  readings. 

Observe  and  record  the  zero  deflection  of  each  instrument  be- 
fore connecting  it  into  the  circuit,  also  record  the  correction  con- 
stant of  each  instrument  on  the  data  sheet.  If  this  constant  is 
appreciable,  each  observation  should  be  duly  corrected.  Note  on 
the  data  sheet  the  number  of  each  instrument  used  somewhat  as 
follows : 

"Weston  Voltmeter  No.  65. 
Zero  deflection,  0.5  volt;  Correction  constant,  ±0.97. 

5.  Rheostats. — A    suitable   rheostat   or   adjustable    resistance 
should  be  employed  in  most  cases,  both  as  a  protection  for  circuits 
of  low  resistance  and  to  afford  the  required  adjustments  of  the 
current  throughout  the  experiment.     Always  select  a  rheostat 


INTRODUCTION  5 

that  will  carry  the  required  current  without  overheating  and  that 
will  give  the  necessary  variations  of  resistance. 

So-called  field  rheostats  designed  for  use  in  series  with  the 
shunt  field  windings  of  generators,  are  adapted  to  low  current 
values,  and  must  not  be  used  where  larger  currents  are  in- 
volved. Look  on  the  rheostat  for  its  current  rating. 

A  bank  of  incandescent  lamps  connected  in  parallel  may  be 
used  as  a  rheostat.  A  lamp  bank  is  a  safe  device  to  use  for  this 
purpose  because  the  resistance  cannot  be  reduced  to  zero.  Note 
that  when  one  lamp  is  turned  on  the  resistance  is  twice  as  great 
as  when  two  are  on.  In  other  words,  the  more  the  lamps  turned 
on  the  less  the  resistance.  Each  16  candle-power  carbon  lamp 
when  burning  has  a  resistance  of  about  220  ohms  and  carries  one- 
half  an  ampere  at  110  volts. 

Other  types  of  rheostats  for  special  purposes  are  usually  avail- 
able in  the  instrument  rooms. 

6.  Circuit    Breakers    and    Fuses. — When    a    circuit    breaker 
opens,  always  open  the  main  switch  before  closing  the  breaker, 
then  close  the  switch.    If  trouble  is  still  on  the  line,  the  breaker 
is  apt  to  open  when  the  switch  is  closed. 

If  a  fuse  blows,  open  the  main  switch  before  replacing  the  fuse, 
and  after  the  new  fuse  has  been  installed  close  the  main  switch. 
See  that  the  fuse  is  of  the  proper  size  for  the  circuit  it  is  to  pro- 
tect. 

7.  Constant  and  Variable  Factors  in  the  Experimental  Work. 
—In  many  of  the  experiments,  observations  are  made  of  the 
changes  which  take  place  in  one  or  more  factors  when  certain 
conditions  are  maintained  constant  and  certain  other  factors  are 
varied  according  to  some  prescribed  method.     This  gives  rise  to 
three  classes  of  observation  items,  namely,  constants,  independent 
variables  and  dependent  variables. 

For  example,  in  Experiment  1,  under  item  1,  Order  of  Work, 
with  a  given  size  and  kind  of  wire  and  a  constant  current  flowing, 
the  lengths  of  the  wire  are  to  be  varied  and  the  changes  in  the 
volts  drop  for  these  different  lengths  are  to  be  observed.  Here 
the  constants  are  the  size  and  kind  of  wire  and  the  current ;  the 
independent  variable,  that  which  is  changed  directly  by  the  ope- 
rator, is  the  length  of  the  wire ;  and  the  dependent  variable  is  the 
volts  drop. 


6  LABORATORY  MANUAL 

In  each  experiment,  the  student  is  to  indicate  on  the  data 
sheet,  preferably  at  the  top  of  the  various  columns,  to  which  of 
these  classes  of  observation  items  the  data  belongs. 

8.  Suggestions  for  the  Work  in  the  Laboratory. — The  fore- 
man of  each  group  of  men  is  to  prepare  a  ruled  and  labelled  data 
sheet  on  the  paper  for  that  purpose  before  the  work  of  taking 
observations  is  begun.  There  is  to  be  one  data  sheet  for  each 
man  in  the  group  and  one  extra  copy  is  to  be  made  for  placing  on 
file  with  the  Instructor. 

To  secure  instruments  before  beginning  the  work,  fill  out  a  re- 
quisition card  with  complete  designation  of  each  instrument  de- 
sired. 

Before  disconnecting  the  apparatus,  have  the  data  sheets  ap- 
proved by  the  Instructor  so  that  if  any  observations  are  missing, 
they  may  be  taken  without  the  trouble  of  re-connecting  the  appa- 
ratus. 

At  the  end  of  the  experiment,  return  all  instruments  to  the 
supply  room,  disconnect  all  wires  and  arrange  the  apparatus  in 
the  same  condition  in  which  it  was  found. 

The  usual  order  of  the  work  in  a  given  laboratory  period  may 
be  summarized  as  follows : 

1.  Recitation  on  the  assigned  articles  in  the  text  book  and  on 
the  Theory  in  the  Manual. 

2.  General  inspection  of  the  apparatus  to  be  used. 

3.  Data  sheets  ruled  up  and  the  columns  of  same  labelled  by 
the  foreman  of  the  group. 

4.  Requisition  card  filled  out  and  instruments  secured  from 
the  supply  room. 

5.  Connections  made,  and  inspected  by  the  Instructor  before 
power  is  applied  to  the  apparatus. 

6.  Observations  taken  for  securing  the  required  data. 

7.  Data  sheets  approved  and  stamped  by  the  Instructor  be- 
fore the  connections  are  taken  apart. 

8.  Connections  taken  down,  instruments  returned  to  supply 
room  and  apparatus  put  in  order. 

9.  Carbon  copy  of  data  placed  on  file  with  the  Instructor,  and 
assignment  of  experiment  for  the  following  exercise  ascertained. 

9.  Written  Report. — The  written  Report  is  to  consist  (a)  of 
the  original  data  sheet;  (b)  a  diagram  of  the  connections  em- 


INTRODUCTION  7 

ployed  in  each  item  of  the  given  experiment  showing  the  instru- 
ments on  the  drawing  labelled  with  their  respective  laboratory 
numbers;  (c)  answers  and  calculations  based  on  the  heading 
Written  Report  at  the  end  of  each  experiment  in  the  following 
directions.  These  report  pages  are  to  be  bound  by  a  brass  clip 
into  the  laboratory  cover,  and  on  this  cover  the  blanks  are  to  be 
duly  filled  in  with  the  name  of  the  student,  number  of  the  ex- 
periment, and  so  on. 

The  following  rules  will  be  observed  in  connection  with  the 
written  reports : 

(a)  The  written  report  on  a  given  experiment  is  to  be  handed 
in  one  week  after  the  performance  of  that  experiment,  that  is, 
where  there  is  one  laboratory  exercise  each  week,  the  written  re- 
port is  to  be  handed  in  at  the  laboratory  exercise  next  following 
that  on  which  the  experiment  was  performed. 

(b)  An  extension  of  time  will  be  granted,  provided  a  written 
valid  excuse  is  presented  on  the  form  provided  for  this  purpose, 
on  the  due  date  of  the  report,  approved  by  the  Instructor,  and 
attached  to  the  data  sheet  in  the  report  when  the  latter  is  handed 
in. 

(c)  Reports  handed  in  after  the  expiration  of  two  weeks  from 
the  date  on  which  the  experiment  was  performed  (except  in  case 
of  illness)  will  be  given  reduced  credit. 

NOTE:  The  actual  work  on  the  written  reports  of  the  various  experi- 
ments has  been  reduced  to  a  minimum  consistent  with  an  understand- 
ing of  what  was  done  in  making  the  observations.  The  advantageous 
time  to  make  up  such  a  brief  summary  of  the  work  performed  in  a 
laboratory  exercise  is  within  a  week  after  the  actual  performance  of  the 
work.  Hence  the  preceding  requirement  (a)  stating  that  the  report  is 
due  one  week  from  the  laboratory  exercise  involved. 


Experiments  1  to  20,  inclusive,  constitute 
the  Direct  Current  portion  of  the  Manual. 


DIRECT  CURRENT 

EXPERIMENT  1. 
Ohm's  Law. 

See  articles  48,  49,  50,  52,  53,  73  and  81  in  the  text  book  (Tim- 
bie's  Elements  of  Electricity). 

The  purpose  of  this  experiment  is  to  verify  the  relations  of 
pressure  or  electromotive  force  (E)  in  volts,  current  (7)  in  am- 
peres, and  resistance  (R)  in  ohms  in  a  circuit  as  expressed  by 
Ohm's  law. 

Theory. — Nearly  all  electrical  problems  involve  a  knowledge 
of  Ohm's  law.  This  law  may  be  expressed  by  the  following  equa- 
tion: 

Pressure  Volts  E 

Current=  ^ — r—         .  or  Amperes  =  ~--r      ;    or  /=« 

Resistance ,  Ohms  R 

obviously  this  expression  may  be  written  in  the  following  forms : 

rr 

E  =  RI  and  R= -j. 

In  the  use  of  this  law,  it  is  very  important  to  consider  7,  E  and 
72  in  the  particular  portion  of  the  circuit  involved,  and  further, 
the  law  applies  only  to  that  portion  of  E  which  is  used  in  over- 
coming resistance.  The  voltage  E  measured  between  the  ends  of 
a  simple  wire  or  across  the  terminals  of  a  motor  armature  when 
stationary  may  be  substituted  in  the  formula  for  Ohm's  law  with 
the  corresponding  current,  for  the  calculation  of  the  resistance 
R,  because  all  of  E  is  used  in  overcoming  resistance  in  these  cases. 
In  delivering  current  to  an  electric  motor  in  motion,  however,  a 
portion  only  of  the  pressure  (or  voltage  E)  at  the  terminals  of 
the  motor  armature  is  used  in  overcoming  resistance  in  the  arma- 
ture windings,  the  remainder  of  the  pressure  going  to  overcome 
the  voltage  generated  in  the  armature  and  thus  being  available  as 

9 


10 


LABORATORY  MANUAL 


useful  pressure  in  the  rotation  of  the  machine.  In  this  latter 
case,  the  voltage  recorded  by  a  voltmeter  at  the  terminals  of  the 
armature  cannot  be  used  in  the  formula  for  Ohm  7s  law  because  a 
portion  only  of  this  voltage  is  used  in  overcoming  resistance. 

When  the  resistance  of  a  simple  wire  is  determined  by  measur- 
ing E  and  I  and  substituting  inphm's  law,  the  method  is  known 
as  the  voltmeter-ammeter  method  of  measuring  resistance. 


Supply  Mains  (110  Volts  D  C.) 

A 

r  i         r  i 

Ammeter 


Main  Switch 

dh      ft 


Voltmeter 


Short  Circuiting  / 
Switch       o 


Wires  to  be  Measured 


66666 


Protective  Resistance  (Lamp  Bank) 

Fig.  2. — Apparatus  and  connections  for  measuring  the  resistance  of 
a  wire  by  the  voltmeter-ammeter  method. 

Current  Supply. — 110  volte  Direct  Current. 

Apparatus  Required. —  (1)  Lengths  of  various  sizes  and  kinds 
of  wire;  (2)  protective  resistance;  (3)  yard  stick;  (4)  incan- 
descent lamps  arranged  for  series  or  parallel  connection;  (5) 
fuses;  (6)  suitable  voltmeter  with  flexible  leads;  and  (7)  an  am- 
meter. 

Order  of  Work. — Make  a  diagram  of  the  exact  connections  em- 
ployed in  each  of  the  following  items,  labelling  each  instrument 
on  the  drawing  with  its  laboratory  number. 

1.  Place  a  suitable  protective  resistance  in  series  with  the  am- 
meter and  the  wire  to  be  tested  as  shown  in  Fig.  2.  With  a 


DIRECT  CURRENT 


11 


given  constant  current  flowing  through  the  wire  as  indicated  by 
the  ammeter,  observe  the  volts  drop  across  1/3,  2/3  and  the  en- 
tire length  respectively  of  the  wire  and  observe  the  supply  volt- 
age. Record  these  observations  as  in  Form  1. 


No. 

Supply 

Voltage 

Kind  of 
Wire 

Size  of 
Wire 

Length  of  Wire 
in  Feet 

Amperes 

Volts  Drop 

1 

2 

3 

4 

Form  1. 


2.  With  the  voltmeter  across  the  entire  length  of  the  wire  used 
in  item  1,  observe  the  volts  drop  for  three  different  values  of  cur- 
rent and  record  as  in  Form  1. 


Supply  Mains  (110  Volts  D  C.) 

A 

-4i       r  i— 

^       ^    Voltmeter 

Ammeter 


Fig.  3. — Wiring  diagram  for  a  study  of  the  voltage  and  current  re- 
lations in  a  series  circuit,  made  up,  in  this  case,  of  two  incandescent 
lamps  connected  in  series. 

3.  Repeat  items  1  and  2  for  the  second  and  third  samples  of 
wire. 


12 


LABORATORY  MANUAL 


4.  With  two  lamps  in  series  with  the  ammeter  as  in  Fig.  3,  ob- 
serve the  current  and  volts  drop  across  each  lamp  and  the  supply 
voltage,  using  Form  2. 


Supply  Mains  (110  Volts  D  C.) 

A^ 

—11          f  T— 

Voltmeter 

^•~    ^7                                Ammeter 

r  --, 

Lamp  Bank  (Lamps  in  Parallel) 


Fig.  4.  —  Diagram  for  a  study  of  the  voltage  and  current  relations 
in  a  parallel  circuit,  made  up,  in  this  case,  of  several  incandescent 
lamps  in  parallel. 

5.  With  the  lamps  connected  in  parallel,  all  the  lamps  turned 
off  except  one,  and  an  ammeter  in  series  with  this  one  lamp  as 
shown  in  Fig.  4,  observe  the  current  in  and  the  volts  drop  across 
this  lamp.  With  the  same  lamp  turned  on,  repeat  for  a  second 
lamp,  observing  the  current  in  the  second  lamp  by  the  difference 
in  the  readings  of  the  two  ammeters.  Use  Form  2. 


No. 

Circuit 
Involved 

Supply 
Amperes 

Amperes  in 
One  Lamp 

Supply 
Voltage 

Volts  Across 
One  Lamp 

1 

2 

3 

Form  2. 

Written  Report. — The  written  report  is  to  consist  of  the  origi- 
nal data,  and  a  diagram  of  connections  as  specified  in  Article  9 
of  the  Introduction,  together  with  answers  to  the  following  ques- 
tions. 

1.  On  what  does  the  resistance  of  a  wire  depend  ? 


DIRECT  CURRENT  13 

2.  On  what  does  the  voltage-drop  in  a  wire  depend  ? 

3.  Explain  briefly  the  results  under  item  1,  Order  of  Work, 
and  calculate  the  resistance  in  Ohms  for  each  of  the  three  obser- 
vations. 

4.  Explain  briefly  the  results  under  item  2,  Order  of  Work, 
and  calculate  the  resistance  in  Ohms  for  each  of  the  three  obser- 
vations. 

5.  Same  for  items  3  and  4,  Order  of  Work. 

6.  Prom  the  observations  taken,  what  effect  has  the  size  and 
kind  of  wire  on  the  resistance  ? 

7.  What  distinguishes  the  relations  of  voltage  and  current  in 
the  component  parts  of  series  and  parallel  circuits  to  the  supply 
voltage  and  current  ? 


EXPERIMENT  2. 

Measurement  of  Armature  and  of  Field  Resistance. 

See  Articles  75,  81  and  115  in  the  text  book  (Timbie's  Ele- 
ments of  Electricity). 

The  purpose  of  this  experiment  is  to  measure  the  resistance  of 
the  armature  and  of  the  shunt  and  series  field  windings  of  a  gen- 
erator by  the  voltmeter-ammeter  method. 

That  portion  of  a  circuit  carrying  relatively  high  current 
usually  has  a  low  resistance  in  order  to  reduce  the  losses  in  the 
circuit.  Hence,  an  armature  and  a  series  field  of  a  generator, 
through  which  the  main  part  of  the  generated  current  flows,  have 
low  resistance  windings,  while  the  shunt  field  winding,  through 
which  a  relatively  small  current  flows,  has  a  high  resistance 
winding. 

Current  Supply. — 110  volts  Direct  Current. 

Apparatus  Required. — (1)  A  compound  generator;  (2)  pro- 
tective resistance  (lamp  bank),  and  (3)  a  suitable  ammeter  for 
the  armature  and  series  field  winding;  (4)  an  adjustable  resist- 
ance (field  rheostat),  and  (5)  a  suitable  ammeter  for  the  shunt 
winding;  and  (6)  suitable  voltmeters  with  flexible  leads  for  the 
three  sets  of  observations. 


14 


LABORATORY  MANUAL 


Order  of  Work. — Make  a  diagram  of  the  exact  connections  em- 
ployed in  each  of  the  following  items,  labelling  each  instrument 
on  the  drawing  with  its  laboratory  number. 


Supply  Mains  (110  Volts  D.  C.) 

_a_ 

-1  J         IV- 

Voltmeter 

Ammeter 


Protective  Resistance  (Lamp  Bank) 

Fig.  5. — Measurement  of  the  resistance  of  an  armature  when  station- 
ary. In  the  case  here  shown,  the  voltmeter  indicates  the  volts  drop  be- 
tween the  armature  terminals. 

1.  With  a  suitable  protective  resistance  (lamp  bank)  in  series 
with  the  armature  (which  is  at  rest) ,  and  the  ammeter  as  shown  in 
Fig.  5,  observe  the  volts  drop  across  the  armature  of  the  machine 
for  say  25,  50  and  75  per  cent,  of  full  load  current  (see  name 


No. 

Percent,  of  Full 
Rated  Current 

Amperes 

Volts  Drop 

Resistance  in  Ohms 
(To  be  Calculated  Later) 

1 

2 

3 

Form  3. 


plate  on  machine  for  normal  current).  Allow  a  brief  interval 
between  each  of  these  observations  to  note  the  effect  of  tempera- 
ture rise  on  resistance.  Use  Form  3. 


DIRECT  CURRENT  15 

2.  Repeat  with  the  series  field  winding  in  place  of  the  arma- 
ture. 

3.  With  an  adjustable  resistance  (field  rheostat)  in  series  with 
the  shunt  field  winding  and  the  ammeter,  observe  the  volts  drop 
across  the  field  terminals  for  the  normal  field  current  (when  the 
normal  machine  voltage  is  impressed  on  the  field)   and  for  ten 
and  twenty  per  cent,  below  normal  value.     The  shunt  field  cur- 
rent is  so  small  that  a  smaller  ammeter  will  probably  be  required 
than  in  items  1  and  2. 

Written  Report. — 1.  What  is  the  resistance  of  the  armature  for 
each  of  the  three  values  of  current  ?    What  is  the  average  ? 

2.  Same  for  the  series  field  winding  ? 

3.  Does  the  resistance  increase  for  the  higher  values  of  current 
in  1  and  2,  and  if  so,  why  ? 

4.  What  is  the  resistance  of  the  shunt  field  winding  for  each 
of  the  three  values  of  current  ?    What  is  the  average  V 

5.  Why  is  Ohm's  law  applicable  to  the  finding  of  the  resist- 
ance in  each  of  these  three  cases  ? 

6.  Would  Ohm's  law  apply  if  the  armature  in  item  1  were  in 
motion  ?    Explain  briefly. 


EXPERIMENT  3. 
Voltage  and  Power  Losses  in  Transmission. 

See  Articles  45,  46,  47,  61,  62  and  69  in  the  text  book  (Timbie's 
Elements  of  Electricity). 

The'  purpose  of  this  experiment  is  to  gain  a  working  knowledge 
of  the  calculation  of  volts  drop  and  power  loss  in  the  transmission 
of  electric  power  from  one  place  to  another. 

Theory. — In  the  transmission  of  electric  power  (El)  over 
longer  or  shorter  distances,  the  current  /  determines  the  loss  in 
voltage  and  the  loss  of  power  for  a  given  size  of  transmission 
wire.  Thus,  if  the  resistance  R  of  the  two  wires  out  and  back  be 
known,  the  product  RI  indicates  the  volts  drop  in  the  line  (out 
and  back),  and  the  product  RP  indicates  the  power  loss  (watts) 
in  transmission.  In  the  simple  apparatus  shown  in  Fig.  6  (see 
Fig.  97,  Timbie),  the  two  wires  leading  from  the  supply  switch 


16 


LABORATORY  MANUAL 


to  the  lamps  serve  on  a  small  scale  as  transmission  wires,  conduct- 
ing the  electric  current  from  the  supply  switch  to  the  lamps. 


Supply  Mains  (110  Volts  D.  C.) 

—  r 

JL 

i      [)—  ' 

X"N    . 

Main  Switch 


Voltmeter 


Flexible  Leads 


-Oi 

•o 
-o 

o 
o 


Short  Transmission  Line 


Fig.  6. — Measurement  of  the  voltage  and  power  losses  in  a  short 
transmission  line.  The  voltmeter,  as  here  shown,  indicates  the  volts 
drop  in  one  side  of  the  lin'e  only. 

Current  Supply.— 110  volts  Direct  Current. 

Apparatus  Required. —  (1)  Long  wires  across  the  r0om  ar- 
ranged as  in  Fig.  6;  (2)  a  low  and  (3)  a  high  reading  voltmeter 
with  flexible  leads;  and  (4)  an  ammeter. 


i. 

1 
P 

& 

i 

•8 

6 
& 

So 

00 

1 
1 

00 

C 

1  & 

E-    -2 

8-2 

| 
1 

«< 

VoHs  Drop 

These  Line  Calculations 
to  be  Made  Later 

C 

1! 

O  o 

Is 

£3 
S* 

1 

03  e 
^=J 

|o 

Resistance 
in  Ohms 

o. 

eg 

la 

1" 

l 

2 

3 

Form  4. 


Order  of  Work. — Make  a  diagram  of  the  exact  connections  em- 
ployed in  each  of  the  following  items,  labelling  each  instrument 
on  the  drawing  with  its  laboratory  number. 


DIRECT  CURRENT  17 

1.  Measure  the  current  in  each  wire  separately,  which  connect 
the  lamps  to  the  supply  switch,  also  the  volts  drop  in  these  two 
wires  (all  the  lamps  being  turned  on),  from  which  the  resistance 
R  of  the  wires  may  be  calculated  by  Ohm's  law.    Use  Form  4. 

2.  Measure  the  supply  voltage  and  the  voltage  at  the  lamp 
terminals  for  a  constant  value  of  current  (all  the  lamps  turned 
on),  recording  these  observations  in  Form  4. 

3.  Repeat  1  and  2  when  half  of  the  lamps  are  turned  off,  using 
Form  4. 

Written  Report. — 1.  Calculate  the  resistance  R  for  each  of  the 
wires  in  1,  Order  of  Work,  and  calculate  the  total  RI  (volts) 
drop  and  the  total  RP  (watts)  loss  in  the  two  line  wires. 

2.  Find  the  RI  drop  in  the  line  wires  for  2,  Order  of  Work,  by 
subtracting  the  voltage  at  the  lamp  terminals  from  the  supply 
voltage. 

3.  Calculate  the  resistance  R  for  each  of  the  wires  in  3,  Order 
of  Work,  and  calculate  the  total  RI  (volts)  drop  and  the  total 
RP  (watts)  loss  in  the  line  wires  for  half  the  lamps  in  operation. 

4.  How  does  the  RI  drop  in  1  and  in  2  compare  ? 

5.  How  does  the  RI  drop  and  the  RP  loss  in  1  and  in  3  com- 
pare ?    Explain  briefly. 


EXPERIMENT  4. 
Study  of  Measuring  Instruments. 

See  Articles  57,  58,  59,  60,  61,  62,  63  and  64  in  the  text  book 
(Timbie's  Elements  of  Electricity),  also  Articles  3  and  4  in  the 
introduction  to  the  Manual. 

The  purpose  of  this  experiment  is  to  gain  an  understanding, 
both  as  regards  construction  and  operation,  of  the  features  of 
electrical  instruments  for  the  measurement  of  current,  voltage 
and  power. 

Theory. — The  underlying  principle  on  which  a  majority  of  in- 
struments is  based,  is  the  mechanical  force  produced  on  a  wire 
carrying  current  when  the  wire  is  placed  in  a  magnetic  field. 
This  mechanical  force  is  directly  proportional  to  the  current  for 


18 


LABORATORY  MANUAL 


a  given  value  of  the  magnetic  field  and  for  a  given  length  of  wire. 
Hence,  by  the  introduction  of  a  balancing  hair  spring,  the  motion 
of  the  pivoted  wire  and,  hence,  that  of  the  attached  needle  or 
pointer  is  proportional  to  the  current  strength. 

In  the  ammeter  the  scale  of  the  instrument  is  calibrated  in  am- 
peres; in  the  voltmeter,  the  needle  is  actuated  by  the  current 
through  the  movable  coil,  but  the  scale  is  calibrated  in  volts  (this 
is  possible  since  E=I/R  where  R  is  the  constant  resistance  of  the 
instrument). 


Fig.  7. — Diagram  for  observing  the  mechanical  force  on  an  electric 
wire  when  placed  in  a  magnetic  field. 

In  the  wattmeter  the  magnetic  component  of  the  mechanical 
force  is  proportional  to  the  main  current  since  the  magnetism 
may  be  considered  as  produced  by  the  main  current  through  a 
stationary  coil,  while  the  movable  coil  carries  a  current  value 
which  is  proportional  to  the  voltage  E  across  the  terminals  of  the 
instrument,  therefore,  the  deflection  of  the  needle  is  proportional 
to  the  product  El.  Hence,  the  wattmeter  is  a  combined  ammeter 
and  voltmeter. 

In  the  watt-hour  meter,  the  principle  is  that  of  a  small  motor 
in  which  the  force  and  the  number  of  revolutions  per  second  is 
proportional  to  El,  and  the  total  number  of  revolutions  in  a  given 
time  t  and,  hence,  the  displacement  of  the  pointers  on  the  dials 


DIRECT  CURRENT  19 

is  proportional  to  the  product  Elt.  The  watt-hour  meter  is  used 
in  residence  electric  lighting  work  for  recording  the  number  of 
watt-hours  consumed  per  month. 

Current  Supply. — 110  volts  Direct  Current. 

Apparatus  Required. —  (1)  A  length  of  small  sized  wire;  (2) 
a  protective  resistance  (lamp  bank)  ;  (3)  a  strong  electromagnet; 
(4)  a  Siemens  Electrodynamometer ;  (5)  a  board  on  which  are 
mounted  the  parts  of  a  Weston  instrument;  (6)  a  Weston  am- 
meter (10  amperes  range)  ;  (7)  a  Weston  voltmeter  (150  volts 
range)  ;  (8)  a  wattmeter  (5  amperes,  150  volts  range)  ;  and  (9) 
a  watt-hour  meter. 

Order  of  Work. — Make  a  diagram  of  the  exact  connections  em- 
ployed in  each  of  the  following  items  which  make  use  of  the  elec- 
tric current,  labelling  each  instrument  on  the  drawing  with  its 
laboratory  number. 

1.  Connect  the  length  of  small  sized  loosely  stretched  wire  in 
series  with  an  ammeter  and  a  protective  resistance  (lamp  bank), 
also  connect  the  electromagnet  in  series  with  a  lamp  bank  and  a 
second  supply  switch  as  shown  in  Fig.  7.    "With  the  current  of 
about  five  lamps  flowing  through  the  loose  wire,  quickly  close 
the  supply  switch  of  the  magnet.    Note  the  mechanical  force  on 
the  wire.    Open  and  close  the  switch  of  the  electromagnet  a  num- 
ber of  times  in  succession.    Repeat  with  the  current  of  one  lamp 
and  of  ten  lamps  flowing  through  the  loose  wire. 

2.  Connect  the  Siemens  Electrodynamometer  in  series  with  a 
lamp  bank  to  the  line  and  note  the  action  of  the  movable  coil 
when  a  current  flows  through  the  instrument;  reverse  the  cur- 
rent and  note  the  direction  of  the  force  as  compared  to  the  direc- 
tion in  the  preceding  case.     (Caution:  do  not  exceed  the  current 
rating  of  the  instrument.)     Note  carefully  what  produces  the 
mechanical  force  on  the  movable  coil. 

3.  On  the  board  containing  the  parts  of  a  Weston  instrument 
note  and  make  a  simple  sketch  of  the  magnet  and  of  the  movable 
coil.    Look  for  these  parts  through  the  glass  in  the  cover  of  one 
of  the  Weston  instruments. 

4.  Connect  a  lamp  bank  and  the  ammeter,  voltmeter,  watt- 
meter and  watt-hour  meter  as  shown  in  Fig.  8.    Turn  on  a  por- 
tion of  the  lamps  and  observe  the  reading  of  the  watt-hour  meter 


20 


LABORATORY  MANUAL 


before  and  say  15  minutes  after  turning  on  the  lamps.  Observe 
and  record  the  ammeter,  voltmeter  and  wattmeter  readings  sev- 
eral times  during  this  15  minute  interval.  Use  Form  5. 

Before  disconnecting  this  apparatus  open  the  supply  switch, 
reverse  the  terminals  of  the  ammeter  and  close  the  switch  mo- 
mentarily. Note  the  effect  011  the  direction  of  force  as  shown  by 
the  movement  of  the  needle. 


Supply  Mains  (110  Volts  D.  C.) 

^ 

A 

—  r  i       ft— 

Ammeter 


[  I         ( 


Main  Switch 


Watthour 
Meter 


Wattmeter 


Flexible  Leads 


Fig.  8. — Measurement  of  the  power  supplied  to  a  bank  of  lamps. 
The  flexible  lead  from  the  watthour  meter  is  one  voltage  connection, 
the  other  is  made  in  the  instrument  to  the  wire  carrying  the  main  cur- 
rent. 

Written  Report. — 1.  What  effect  was  produced  by  the  larger 
as  compared  with  the  smaller  current  values  through  the  loosely 
stretched  wire  on  the  mechanical  force  in  item  1,  Order  of  Work  ? 
What  was  the  function  of  the  electromagnet  in  this  particular 
test? 

2.  What  produces  the  mechanical  force  in  the  Siemens  Electro- 
dynamometer?     What  effect  was  produced  on  the  direction  of 
this  mechanical  force  by  reversing  the  current  through  the  in- 
strument ?    Why  ? 

3.  From  the  results  obtained  under  item  4,  Order  of  Work, 
calculate  the  watt-hours  from  the  voltmeter  and  ammeter  read- 
ings, and  from  the  wattmeter  readings  and  compare  each  of  these 


DIRECT  CURRENT 


21 


results  with  the  number  of  watt-hours  recorded  by  the  watt-hour 
meter. 

4.  If  the  commercial  rate  for  electric  power  in  residence  light- 
ing is  9  cents  per  1000  watt-hours  (9  cents  per  kilowatt-hour), 
then,  in  a  residence  containing  twenty  25-watt  tungsten  lamps, 
how  many  hours  can  all  the  lamps  be  turned  on  per  night  for  a 
month  of  30  days  to  make  the  bill  equal  $3.60  for  the  month? 

5.  What  effect  was  produced  on  the  direction  of  throw  of  the 
ammeter  needle  in  item  4,  Order  of  Work,  when  the  connections 
were  reversed?     Compare  this  result  with  that  of  the  direction 


£ 

| 

S 

1 

1 

Watthour  Meter 
Readings 

Watthours  for  the  Interval 
(To  be  Calculated  Later) 

From  Watthour 
Meter  Readings 

From  Voltmeter 
Ammeter  Readings 

From.  Wattmeter 
Readings 

i 

2 

3 

Form  5. 


of  force  on  the  Electrodynamometer  when  the  current  was  re- 
versed in  its  coils  ?    Explain  the  difference  briefly. 


EXPERIMENT  5. 
Study  of  Fuses  and  Circuit  Breakers. 

See  Article  36  in  the  text  book  (Timbie's  Elements  of  Elec- 
tricity), also  Article  6  in  the  Introduction  to  the  Manual. 

The  purpose  of  this  experiment  is  to  gain  familiarity  with  the 
make-up  of  various  types  of  fuses,  the  elements  of  fuse  blocks, 
and  the  construction  and  operation  of  circuit  breakers. 


22  LABORATORY  MANUAL 

Theory. — The  simple  fuse  is  an  alloy  with  a  relatively  low  melt- 
ing point.  A  fuse  is  inserted  in  a  circuit,  usually  one  on  each  side 
of  the  line,  as  a  protection  against  overloads  and  short  circuits, 
which  might  otherwise  injure  the  wires  or  appliances  in  the  cir- 
cuit. Fuses  may  roughly  be  classified  either  as  open  or  enclosed. 
In  the  enclosed  type,  commonly  known  as  the  cartridge  fuse,  the 
fuse  wire  is  surrounded  with  a  fire-proof  covering  which  prevents 
the  scattering  of  small  particles  of  hot  fuse  metal  in  case  of  a 
blow-out. 

The  distance  between  the  terminals  on  standard  fuses  depends 
on  the  voltage,  that  is,  with  higher  voltages  the  fuse  terminals 
are  made  farther  apart  so  as  to  reduce  the  likelihood  of  the  cur- 
rent arcing  across  the  terminals  when  the  fuse  is  blown.  It  is 
obviously  very  important  in  fusing  up  a  circuit,  to  use  a  fuse 
so  rated  as  to  protect  the  circuit  against  current  values  in  excess 
of  the  permissable  limiting  current  for  the  circuit.  Thus,  if  the 
maximum  allowable  current  is  10  amperes,  the  rating  of  the  fuse 
should  not  exceed  this  current  value.  Fuses  are  usually  so  rated 
as  to  allow  a  small  margin  above  the  rating  before  the  fuse  act- 
ually blows.  If  the  circuit  is  opened  by  the  blowing  of  a  fuse, 
an  inspection  will  often  show  whether  the  blow-out  has  been  due 
to  a  simple  overloading  of  the  circuit  or  to  a  short  circuit.  If  a 
slight  overload,  the  fuse  is  melted  away  without  the  appearance 
of  burns  on  the  porcelain  fuse  block.  If  a  short  circuit,  the 
charred  or  burned  appearance  of  the  fuse  block  generally  indi- 
cates the  fact.  The  inspection  here  mentioned  refers  mainly  to 
the  open  fuse. 

The  circuit  breaker  is  an  electromagnetic  device  which  me- 
chanically opens  the  circuit  when  the  current  exceeds  the  value 
for  which  it  is  adjusted  or  set  to  open.  The  continual  breaking 
of  the  circuit  between  metal  surfaces  would  soon  cause  excessive 
wear  due  to  the  arcing,  and  to  avoid  this,  the  actual  opening  of 
the  circuit  is  between  carbon  blocks  which  may  readily  be  re- 
newed as  necessary.  "When  closed,  the  current  is  conducted 
through  copper  contacts,  the  breaker  being  so  arranged  that  as 
it  opens  the  copper  contacts  separate  first  and  then  the  carbon 
contacts. 

Current  Supply. — 110  volts  Direct  Current. 


DIRECT  CURRENT  23 

Apparatus  Required. —  (1)  A  low  and  a  high  voltage  fuse 
block;  (2)  a  ceiling  rosette;  (3)  several  lengths  of  3  to  5  ampere 
fuse  wire;  (4)  a  cartridge  fuse  for  low  current  and  one  for  heavy 
current;  (5)  an  Edison  fuse  plug;  (6)  several  typical  circuit 
breakers;  (7)  an  adjustable  rheostat;  (8)  a  lamp  bank;  (9)  foot 
rule;  and  (10)  an  ammeter. 

Order  of  Work. — 1.  Make  a  simple  sketch  of  the  fuse  blocks  for 
low  and  for  high  voltage  and  of  the  ceiling  rosette,  showing  the 
fuse  as  well  as  the  line  terminals,  and  give  dimensions  on  the 
sketch.  Insert  fuses  in  each  of  the  three,  and  have  them  checked 
by  the  Instructor. 

2.  Connect  the  lamp  bank  in  series  with  the  adjustable  rheo- 
stat and  the  ammeter  through  a  fuse  block  to  the  supply  switch. 
Starting  with  zero  current,  gradually  increase  the  current  until 
the  fuse  blows,  observing  the  value  of  the  current  just  before  the 
circuit  is  broken.     Eepeat  this  test  with  the  fuse  entirely  clear 
of  the  porcelain  block  except  at  the  terminals,  also  when  the 
fuse  rests  against  the  porcelain  throughout  its  length.     (The  fuse 
block  may  be  equipped  with  a  small  sized  fuse  wire  for  this  test, 
a  3  or  5  ampere  fuse  being  sufficient.) 

3.  Make  a  simple  sketch  of  a  low  and  a  heavy  current  cartridge 
fuse.    Take  out  and  replace  each  of  these  fuses,  observing  care- 
fully how  they  make  contact  at  the  terminals.    Measure  the  sup- 
ports and  the  length  of  the  fuse  in  each  case,  recording  the  di- 
mensions on  the  sketches. 

4.  Make  a  simple  sketch  of  an  Edison  fuse  plug  and  its  recep- 
tacle with  dimensions. 

5.  Sketch  two  types  of  circuit  breakers.     Note  how  they  are 
adjusted  for  opening  the  circuit  at  various  current  values,  and 
note  the  carbon  blocks  between  which  the  circuit  is  broken,  and 
the  copper  contacts  between  which  the  current  flows  when  the 
breaker  is  closed. 

6.  Arrange  to  supply  current   to  a  bank  of  lamps  through 
an  ammeter  and  one  of  the  circuit  breakers.    Set  the  breaker  for 
a  nominal  current  value  and  gradually  increase  the  current  by 
turning  on  the  lamps  until  the  breaker  opens.    With  the  lamps 
turned  on  attempt  to  close  the  breaker.     With  the  lamps  still 
on,  open  the  main  switch,  close  the  breaker  and  then  close  the 
main  switch.     This  is  the  regular  procedure  in  closing  a  circuit 


24  LABORATORY  MANUAL 

breaker.     (Caution:  Do  not  exceed  the  capacity  of  the  ammeter 
in  this  test.) 

Written  Report. — 1.  What  difference  was  observed  in  the  dis- 
tance between  fuse  terminals  in  the  fuse  blocks  for  low  and  high 
voltage?  What  is  the  object  of  the  longer  distance  for  the  higher 
voltage  ? 

2.  In  item  2,  Order  of  Work,  was  there  a  fixed  value  of  current 
at  which  the  3  or  5  ampere  fuse  melted?     What  effect  did  the 
fuse  touching  or  not  touching  the  porcelain  have  on  the  current 
value  at  which  the  fuse  melted  ?    Explain  briefly. 

3.  What  advantage  has  the  cartridge  and  Edison  fuse  over  the 
open  fuse?    Has  the  open  fuse  any  advantages ?    What  provision 
is  made  in  the  cartridge  fuse  for  heavy  currents  ? 

4.  State  briefly  the  operation  features  of  the  circuit  breaker. 
Why  are  carbon  contacts  better  than  copper  at  the  opening  point 
of  the  breaker  ?    Why  is  the  breaker  arranged  so  that  the  current 
flows  through  copper  contacts  rather  than  carbon  after  it  is 
closed  ? 

5.  What  effect  was  noticed  when  the  attempt  was  made  to  close 
the  breaker  in  item  6,  Order  of  Work,  before  opening  the  main 
switch  ? 

6.  What  distinguishes  the  blowing  of  a  fuse  or  the  opening  of  a 
circuit  breaker  under  a  gradual  increase  of  current  on  the  one 
hand,  or  by  the  sudden  application  of  a  heavy  current  on  the 
other  hand  ? 


EXPERIMENT  6. 
Study  of  Motor  Starting  Boxes. 

See  Articles  133,  136,  137,  138,  139,  143  and  145  in  the  text 
book. 

A  majority  of  the  laboratory  tests  involve  the  starting  of  a 
motor.  The  purpose  of  this  experiment  is  to  afford  an  oppor- 
tunity for  a  study  of  the  physical  make-up  of  common  types  of 
starting  boxes  and  of  the  principles  of  motor  starting. 

Theory. — The  armature  winding  of  a  motor  has  a  very  low  re- 
sistance and  if  connected  to  110  or  220-volt  mains  directly  at 
starting  an  excessive  current  will  flow,  which  is  apt  to  damage  the 


DIRECT  CURRENT 


25 


machine.  As  in  all  other  cases  where  low  resistance  devices  are 
connected  to  the  110  or  220-volt  mains,  a  protective  resistance 
must  always  be  connected  in  series  with  the  armature  of  a  motor 
before  connecting  it  to  the  supply  mains.  After  applying  the 
power  to  the  motor  it  speeds  up  and  in  so  doing  a  counter  voltage 
is  set  up  in  the  armature  which  acts  like  resistance  opposing  the 
flow  of  current  from  the  supply  mains.  Hence,  as  the  speed  in- 


Supply  Mains  (110  Volts  D.  C.) 

A 

•«  1       I  M 

Ammeter 


[]        I 


Main  Switch 


• AVWMAA, 


Adjustable  Resistance 

Fig.  9. — Simple  connection  for  starting  a  shunt  motor.  Note  that 
the  shunt  field  circuit  is  entirely  independent  of  the  armature  adjust- 
able resistance.  A  voltmeter  is  to  be  connected  across  the  armature 
terminals. 

creases,  the  protective  resistance  may  be  reduced  and  finally  cut 
out  entirely,  since  at  normal  speed  the  counter  voltage  nearly 
equals  the  applied  voltage  and  the  relatively  small  difference  or 
resultant  voltage  sends  only  a  moderate  current  through  the 
low  resistance  of  the  armature. 

The  simplest  form  of  starting  resistance  consists  of  a  pro- 
tective rheostat  in  series  with  the  armature  as  shown  in  Fig.  9. 
In  this  diagram  note  that  the  shunt  field  winding  is  connected 
directly  to  the  supply  mains  through  a  field  rheostat  and  that  the 


26  LABORATORY  MANUAL 

starting  resistance  has  nothing  whatever  to  do  with  the  field  cir- 
cuit. (The  field  rheostat  merely  serves  to  vary  the  field  current 
during  the  operation  of  the  motor  if  desirable.)  Other  conveni- 
ent modifications  are  made  as  explained  in  the  text  book  articles 
referred  to,  but  the  underlying  principle  in  all  these  starting 
boxes  is  as  shown  in  Fig.  9. 

If  the  field  circuit  is  opened  or  if  the  field  current  is  greatly 
reduced  while  a  motor  is  in  operation,  the  counter  voltage  is 
either  reduced  to  zero  or  greatly  lessened  and  the  resistance  of 
the  armature  being  low,  an  excessive  current  is  apt  to  damage 
the  machine.  As  a  precaution,  therefore,  never  open  the  field  cir- 
cuit when  a  motor  is  running,  and  always  start  with  all  the  re- 
sistance of  the  field  rheostat  cut  out  so  that  the  counter  voltage 
of  the  armature  may  quickly  reach  its  normal  value. 

Current  Supply. — 110  volts  Direct  Current. 

Apparatus  Required. —  (1)  An  adjustable  resistance  which 
will  carry  the  armature  current  of  the  motor  to  be  tested,  without 
overheating;  (2)  a  simple  "no  voltage"  or  "no  field"  release 
starting  box;  (3)  a  starting  box  with  "no  voltage"  and  "over- 
load" release;  (4)  ammeter  and  (5)  voltmeter.  (Note:  The 
current  usually  used  in  starting  an  unloaded  motor  is  larger  than 
its  normal  running  current,  hence,  an  ammeter  must  be  selected 
that  will  carry  the  maximum  starting  current. 

Order  of  Work. — 1.  Connect  the  adjustable  resistance  and  the 
ameter  in  series  with  the  armature  of  the  shunt  motor  assigned, 
while  the  shunt  field  winding  is  to  be  connected  directly  to  the 
supply  mains  through  a  field  rheostat  as  shown  in  Fig.  9.  (Cau- 
tion: Be  sure  that  the  field  circuit  is  always  connected  to  the 
supply  mains  when  current  is  being  delivered  to  the  armature.) 
With  all  the  armature  rheostat  resistance  cut  in  and  all  the  field 
rheostat  resistance  cut  out,  throw  in  the  supply  switch  and  grad- 
ually cut  out  the  armature  resistance  entirely.  Note  the  current 
just  at  starting  and  after  the  motor  is  running  at  full  speed.  Al- 
so note  the  voltage  across  the  armature.  Record  these  observa- 
tions on  the  data  sheet  and  make  a  diagram  of  the  exact  connec- 
tions labelling  each  instrument  on  the  drawing  with  its  labora- 
tory number. 


DIRECT  CURRENT  27 

2.  Repeat  with  a  portion  of  the  field  rheostat  resistance  cut  in, 
noting  carefully  any  difference  in  the  ammeter  and  voltmeter 
readings.    Record  these  observations. 

3.  Take  the  cover  off  one  of  the  starting  boxes  and  trace  out  the 
connections  of  the  armature  portion  of  the  resistance.     Make  a 
diagram  of  these  connections  labelling  each  part  in  terms  of  the 
function  it  plays  in  the  starting  of  the  motor.     Note  also  any 
other  parts  of  the  starting  box  involved. 

4.  Make  a  general  inspection  of  the  "no  voltage"  and  the 
"overload"  release  types  of  starting  boxes,  trace  their  circuits 
and  determine  their  functions,  ascertaining  such  points  as  may 
not  be  clear,  from  the  Instructor. 

Written  Report. — 1.  Why  was  the  armature  current  higher  at 
starting  than  after  the  motor  was  running  at  normal  speed  in 
item  1,  Order  of  Work?  What  determines  the  initial  current? 
Explain  the  difference  in  the  voltmeter  reading  at  starting  and  at 
normal  speed  in  this  same  item. 

2.  In  item  2,  Order  of  Work,  what  effect  was  noticed  on  the 
starting  current  and  voltage  with  the  increased  field  rheostat  re- 
sistance ?    Explain  briefly.    What  da  you  conclude  from  this  ob- 
servation as  to  the  proper  way  for  setting  the  field  rheostat  at 
starting? 

3.  What  is  the  underlying  principle  of  all  starting  boxes  ? 

4.  What  is  the  function  of  the  "no  field"  or  the  "no  voltage" 
release  in  a  starting  box  ? 

5.  Same  for  the  "overload"  release? 


EXPERIMENT  7. 

Study  of  Electric  Lamps. 

See  Articles  214,  215,  218  (two  articles),  219,  220  and  222  in 
the  text  book. 

The  object  of  this  experiment  is,  first,  to  gain  a  working  knowl- 
edge of  the  practical  features  of  construction  and  operation  of 
the  important  electric  lamps  in  common  use;  and,  second,  from 
this  study  of  lamps  to  secure  a  definite  idea  of  the  voltage  re- 
quirements of  electric  circuits  which  supply  power  for  the  opera- 
tion of  lamps  in  practice. 


28  LABORATORY  MANUAL 

Theory. — The  lamps  in  residence  and  commercial  lighting  are 
nearly  always  operated  from  constant  voltage  electric  supply 
mains.  At  the  end  of  the  Table  on  page  378  of  the  text  book, 
a  summary  is  given  of  the  effects  produced  on  carbon  incan- 
descent lamps  by  a  change  of  1  per  cent,  in  the  voltage.  For  ex- 
ample, a  fall  of  1  per  cent,  in  the  voltage  causes  a  loss  of  5  per 
cent,  in  the  candle-power,  hence,  the  importance  of  maintaining 
constant  voltage  at  the  terminals  of  incandescent  lamps. 

In  the  operation  of  a  shunt  generator,  the  voltage  at  the 
terminals  of  the  machine  falls  off  somewhat  as  the  machine  is 
loaded.  If  this  drop  in  the  voltage  is  great  enough  to  pro- 
duce much  decrease  in  the  candle-power  of  the  lamps  supplied 
by  the  generator,  either  the  field  current  or  the  speed  may  be 
varied  as  the  load  changes,  thus  maintaining  a  constant  or  fairly 
constant  terminal  voltage ;  or  a  series  field  winding  may  be  added 
to  the  machine,  thus  making  it  a  compound  generator,  in  which 
the  tendency  of  the  voltage  to  decrease  is  offset  by  the  strength- 
ening of  the  field  due  to  the  current  in  the  series  winding. 

Several  of  the  experiments  on  the  electrical  features  of  genera- 
tors will  emphasize  this  tendency  of  the  voltage  to  fall  as  the 
load  is  increased,  and  it  will  be  well  in  making  these  subsequent 
observations  to  keep  in  mind  why  constant  terminal  voltage  is  a 
most  important  consideration  in  the  distribution  of  electric  power 
for  lighting. 

Among  the  electric  lamps  in  common  use  may  be  mentioned 
the  Tungsten  (or  Mazda)  incandescent  lamp;  the  Mercury  Va- 
por (or  Cooper  Hewitt)  lamp;  the  carbon  filament  incandescent 
lamp;  and  the  arc  lamp.  Under  arc  lamps,  may  be  mentioned 
the  Flaming  Carbon  arc  used  in  some  cases  for  street  lighting 
and  for  commercial  or  factory  lighting;  the  Metallic  Flame  or 
Magnetite  arc  lamp  used  mainly  for  street  lighting;  and  the 
older  Enclosed  Carbon  arc  lamp. 

Current  Supply. — 110  volts  Direct  Current. 

Apparatus  Required. —  (1)  A  board  on  which  are  mounted  the 
parts  of  a  carbon  filament  incandescent  lamp;  (2)  a  Tungsten 
(or  Mazda)  lamp;  (3)  Focusing,  Intensive  and  Extensive  Holo- 
phane  prismatic  reflectors  with  prints  of  the  respective  distribu- 
tion of  light  curves;  (4)  a  form  "0"  and  form  "H"  shade 
holder;  (5)  a  Mercury  Vapor  (or  Cooper  Hewitt)  lamp  with  a 


DIRECT  CURRENT  29 

diagram  of  the  internal  electrical  connections;  (6)  one  or  more 
of  the  principal  types  of  arc  lamps  with  diagram  of  the  internal 
electrical  connections. 

Order  of  Work. — 1.  Make  a  sketch  of  the  various  parts  (label- 
ling each)  involved  in  the  manufacture  of  carbon  incandescent 
lamps. 

2.  Make  a  simple  diagram  of  a  tungsten  lamp  showing  the 
method  employed  in  mounting  the  filament. 

3.  Make  a  simple  sketch  of  the  Focusing,  Intensive  and  Ex- 
tensive   Holophane    Prismatic    reflectors,    showing    the    general 
shape  of  each,  also  the  approximate  distribution  curves  of  each. 

4.  Make  a  sketch  of  the  form  "0"  and  of  the  form  "H"  shade 
holder  giving  dimensions  on  the  sketches. 

5.  Make  a  diagram  of  the  electrical  connections  of  the  Mer- 
cury Vapor  lamp,  recording  the  rating  of  the  lamp  in  volts,  am- 
peres and  watts. 

6.  Connect  the  Mercury  Vapor  lamp  through  a  suitable  rheo- 
stat to  the  supply  mains  and  observe  its  action  at  starting. 

7.  Make  a  diagram  of  the  electrical  connections  of  the  arc 
lamp  (or  lamps)  available,  recording  the  rating  in  volts,  amperes 
and  watts. 

8.  Connect  the  arc  lamp  (or  lamps)  through  a  suitable  rheo- 
stat to  the  supply  mains  and  observe  the  action  at  starting,  also 
the  method  of  feeding  the  carbons  as  they  are  consumed. 

Written  Report. — 1.  Describe  briefly  the  principal  items  in  the 
manufacture  of  carbon  incandescent  lamps. 

2.  What  is  the  function  of  the  vacuum  ? 

3.  Why  is  a  reflector  a  necessary  auxiliary  with  the  Tungsten 
lamp  for  its  most  economical  operation  ? 

4.  For  what  purposes  are  each  of  the  Holophane  reflectors  best 
suited  ? 

5.  Aside  from  supporting  the  reflector,  what  other  function 
has  the  shade  holder  in  connection  with  Tungsten  lamps  ? 

6.  Describe  briefly  the  method  of  operation  of  the  Mercury 
Vapor  lamp. 

7.  Same  for  the  arc  lamp  (or  lamps). 

•  8.  Between  the  generator  in  the  electric  power  station  and  dis- 
tant lamps  there  is  apt  to  be  an  appreciable  voltage  drop  which 
depends  on  the  current  flowing  through  the  lines.  If  the  attend- 


30  LABORATORY  MANUAL 

ant  in  the  power  station  regulates  the  voltage  of  the  generator 
by  a  voltmeter  which  indicates  the  voltage  in  the  station,  how 
is  constant  or  fairly  constant  voltage  assured  at  the  distant  lamps 
in  residence  lighting  work  ? 


EXPERIMENT  8. 

Building  Up  of  Voltage  in  Shunt  Generator. 
See  Articles  117,  118,  119  and  120  in  the  text  book. 

In  many  of  the  laboratory  experiments  as  well  as  in  practical 
generator  operation,  it  is  necessary  to  understand  the  conditions 
which  must  be  met  in  order  that  a  self-excited  generator  may 
build  up  to  its  normal  voltage  at  starting.  The  object  of  this  ex- 
periment is  to  gain  a  working  knowledge  of  these  conditions  and 
how  to  meet  them. 

Theory. — Electric  generators  may  be  classed  as  to  the  produc- 
tion of  the  necessary  magnetic  field  under  the  head  either  of  sepa- 
rate or  self  excitation.  For  separate  excitation,  the  field  winding 
is  connected  directly  to  the  supply  mains  as  shown  in  Fig.  10 
so  that  the  field  current  is  derived  from  a  generator  already  in 
operation,  and  the  voltage  of  the  machine  rises  to  its  normal  value 
as  soon  as  the  armature  is  brought  up  to  its  normal  speed  because 
the  magnetism  is  at  its  full  value  at  the  start.  In  this  case  the 
field  magnetism  is  practically  independent  of  the  operation  of 
the  machine,  being  dependent  on  the  voltage  of  the  supply  mains 
and  on  the  hand  manipulation  of  the  field  rheostat. 

For  the  self  excitation  of  a  shunt  generator,  the  field  winding 
is  connected  to  the  armature  terminals  through  a  field  rheostat 
as  shown  in  Fig.  10,  and  the  voltage  of  the  machine  even  after 
the  armature  is  brought  up  to  normal  speed  may  sometimes 
amount  only  to  the  several  volts  due  to  residual  magnetism  in 
the  field  poles,  that  is,  to  the  small  amount  of  magnetism  left 
over  from  the  last  time  the  machine  was  in  operation.  Since 
the  field  current  and,  hence,  the  magnetism  depends  on  the  volt- 
age induced  in  the  armature  in  this  case,  and  since  the  voltage 
itself  is  dependent  on  the  field  magnetism  produced  by  the  field 
current,  it  is  obvious  that  the  generation  of  electromotive  force 
must  be  cumulative,  starting  from  the  few  volts  produced  by 


DIRECT  CURRENT 


31 


residual  magnetism  and  rising  to  the  normal  voltage  of  the  ma- 
chine. 

It  will  further  be  obvious  that  no  electromotive  force  can  be 
induced  in  a  self-excited  generator  (connected  for  self  excita- 
tion) unless  there  be  a  small  amount  of  residual  magnetism  in 
the  field  poles  to  begin  with.  When  machines  are  in  regular 
operation  with  an  occasional  shut-down,  there  is  usually  sufficient 


Field  Rheostat 


Separate  Excitation 


Self  Excitation 


Fig.  10. — Two  methods  of  exciting  the  field  of  a  shunt  generator: 
(a)  separate  excitation  is  shown  to  the  left;  and  (b)  self  excitation  to 
the  right. 

residual  magnetism  in  the  fields  for  the  machine  to  build  up  each 
time  it  is  started.  In  practice,  direct  current  generators  are 
usually  operated  with  self  excitation. 

The  principal  condition  to  be  met  for  building  up  is  that  the 
electromotive  force  produced  at  starting  by  the  rotation  of  the 
armature  in  the  weak  residual  magnetism  be  such  that  the  cur- 
rent it  produces  in  the  field  winding  shall  aid  or  increase  the  re- 
sidual magnetism.  If  this  condition  is  fulfilled,  the  increased 
magnetism  produces  an  increased  electromotive  force  which,  in 
turn,  produces  an  increased  field  and  the  electromotive  force  soon 
rises  to  its  full  normal  value  being  limited  by  the  saturation  of 
the  field  magnet  iron. 

If  the  machine  fails  to  build  up,  the  necessary  favorable  con- 
ditions can  usually  be  secured  either  by  reversing  the  connection 


32 


LABORATORY  MANUAL 


of  field  winding  to  armature  terminals  or  by  running  the  arma- 
ture in  the  opposite  direction. 

Other  causes  which  sometimes  affect  the  building  up  of  a  ma- 
chine may  be  due  to  poor  contact  of  the  brushes  on  the  commuta- 
tor, to  the  fact  that  the  field  rheostat  resistance  is  all  cut  in,  or 
to  too  little  residual  magnetism. 

Current  Supply. — 110  volts  Direct  Current. 

Apparatus  Required. —  (1)  A  shunt  generator;  (2)  reversing 
switch  for  readily  interchanging  the  connections  of  field  winding 
to  armature  terminals;  and  (3)  a  voltmeter  with  a  range  slightly 
above  the  rating  of  the  generator. 


Voltmeter 


Switch 

P    f                           ®. 

^ 

s 

Fig.  11. — The  reversing  switch  is  a  convenient  means  for  reversing 
the  terminals  of  the  field  winding  as  connected  to  the  armature  termi- 
nals. (The  student  should  trace  the  circuit  for  the  two  positions  of 
the  reversing  switch  to  determine  how  the  connections  are  reversed.) 

Order  of  Work. — Make  a  diagram  of  the  exact  connections  for 
the  following  items  labelling  each  instrument  with  its  laboratory 
number  on  the  drawing. 

If  the  machine  fails  to  build  up  in  all  of  the  following  cases 
on  first  or  second  trial,  temporarily  disconnect  the  field  from  the 
reversing  switch  and  connect  it  to  the  supply  .mains  for  a  short 
time  so  a-s  to  insure  the  necessary  residual  magnetism  in  the 
field  magnets.  Then  proceed  as  directed  under  the  following 
heads : 

1.  Arrange  the  assigned  shunt  generator  for  self  excitation  as 
shown  in  Fig.  11.  The  reversing  switch  inserted  between  the 
field  winding  and  the  armature  terminals  provides  a  convenient 
means  for  reversing  the  connections  of  field  to  armature. 


DIRECT  CURRENT 


33 


2.  "With  the  field  disconnected  (reversing  switch  open)  drive 
the  machine  at  normal  speed  and  observe  the  electromotive  force 
at  the  armature  terminals  produced  by  residual  magnetism.    Re- 
cord the  electromotive  force  (volts)  and  the  speed  as  in  Form  6, 

3.  Throw  in  all  the  field  rheostat  resistance,  close  the  reversing 
switch  to  the  arbitrary  position  "A"    (this  letter  should  be 
marked  in  chalk  on  one  end  of  the  switch  for  reference)  and  with 
the  armature  rotating  at  normal  speed  in  a  forward1  direction, 
gradually  cut  out  the  field  rheostat.    Observe  the  initial  and  final 
values  of  electromotive  force  produced,  that  is,  before  and  after 


Open 


A" 


Forward 


Out 


Form  6. 

cutting  out  the  field  rheostat  resistance,  the  speed,  and  the  posi- 
tion of  the  reversing  switch  as  in  Form  6. 

4.  Same,  throwing  the  reversing  switch  in  the  opposite  direc- 
tion  (mark  this  second  position  on  the  other  end  of  the  switch 
"B").     Note  that  this  second  position  of  the  reversing  switch 
changes  the  connections  of  field  to  armature. 

5.  Same  as  3  with  armature  rotating  in  a  backward  direction. 

6.  Same  as  4  with  armature  rotating  in  a  backward  direction. 

7.  Note  carefully  what  effect  is  produced  on  the  building  up  in 
those  cases  where  the  conditions  are  favorable  for  building  up, 
except  that  the  field  rheostat  is  all  cut  in. 

Written  Report. — 1.  To  what  is  the  electromotive  force  as  ob- 
served in  item  2,  Order  of  Work,  due? 

iThe  terms  forward  and  backward  as  referred  to  direction  of  rotation 
are  of  course  arbitrary. 

4 


34  LABORATORY  MANUAL 

2.  In  that  case,  under  items  3  and  4,  Order  of  Work,  where  the 
machine  built  up  to  its  normal  voltage,  explain  briefly  to  what 
the  increase  of  voltage  was  due.     In  the  other  case,  what  pre- 
vented the  machine  from  building  up  ? 

3.  Where  a  shunt  generator  fails  to  build  up,  why  should  re- 
versing the  direction  of  armature  rotation  be  favorable  to  build- 
ing up  ? 

4.  Why   should   excessive   brush   resistance   tend   to   prevent 
building  up  even  when  the  connections  of  field  to  armature  or  the 
direction  of  armature  rotation  are  favorable? 

5.  As  a  summary  of  the  observations,  explain  briefly  the  vari- 
ous steps  which  should  be  taken  if  a  shunt  generator  fails  to  build 
up.    If  the  fault  is  due  to  a  lack  of,  or  insufficient  residual  mag- 
netism, how  could  this  lack  be  determined  by  a  simple  test  ob- 
servation?    What  would  happen  if  one  of  the  field  coils  be  re- 
versed ? 


EXPERIMENT  9. 
Electrical  Features  of  the  Shunt  Generator. 

See  Article  121  in  the  text  book,  also  the  Theory  under  Experi- 
ment 7  in  the  Manual. 

The  object  of  this  experiment  is  (a)  to  make  a  study  of  the  fac- 
tors by  means  of  which  the  voltage  of  a  generator  may  be  varied 
or  controlled;  (b)  to  observe  the  tendency  of  the  terminal  voltage 
to  decrease  with  increasing  output;  and  (c)  to  take  the  observa- 
tions for  the  calculation  of  the  so-called  regulation  of  the  ma- 
chine. 

Theory. — By  the  term  voltage  control  is  meant  the  changing 
of  conditions  exterior  to  the  generator  for  the  purpose  of  main- 
taining some  given  value  of  voltage  at  the  terminals  of  the  ma- 
chine at  various  loads.  Thus,  by  changing  the  field  rheostat  re- 
sistance (by  hand),  the  voltage  of  the  machine  may  be  varied 
over  quite  a  wide  range;  or  by  changing  the  speed  of  the  ma- 
chine (by  changing  the  speed  of  the  driving  engine)  the  volt- 
age may  be  varied. 

On  the  other  hand,  as  the  output  (load)  of  the  shunt  genera- 
tor increases,  for  example,  by  turning  on  more  of  the  lamps  it 
supplies,  the  voltage  (RI)  drop  in  the  armature  increases  (be- 


DIRECT  CURRENT  35 

cause  7  increases)  and  the  terminal  voltage  falls  off.  This  drop 
in  voltage  obviously  depends  on  an  inherent  property  of  the  ma- 
chine, namely,  the  amount  of  fixed  resistance  in  the  armature 
winding.  Changes  which  occur  in  the  terminal  voltage  of  a  gen- 
erator, due  to  inherent  properties,  are  referred  to  as  voltage 
regulation  to  distinguish  them  from  changes  which  are  made  by 
varying  conditions  exterior  to  the  machine,  and  referred  to  as 
voltage  control. 

The  principal  means  for  controlling  the  voltage  of  a  shunt 
generator  are  hand  variations  of  the  field  rheostat  resistance,  and 
changes  in  the  speed  by  variations  in  the  speed  of  the  driving 
engine. 

The  main  items  which  govern  inherent  changes  in  the  voltage 
(that  is,  the  regulation)  are  the  resistance  of  the  armature  wind- 
ing, together  with  slight  magnetic  reactions  in  the  armature 
which  tend  to  decrease  the  effective  magnetic  field  and,  hence,  the 
voltage. 

The  percentage  regulation  of  the  generator  is  defined  as  the 
difference  between  the  full  load  and  the  no-load  voltage  divided 
by  the  full  load  voltage  at  constant  speed  (obviously  this  result 
must  be  multiplied  by  100  to  express  it  as  a  percentage).  Thus, 
if  the  full  load  and  the  no-load  voltages  are,  100  and  110  respect- 
ively, the  regulation  is  equal  to  10  per  cent.  If,  in  this  case, 
the  numerical  value  of  regulation  be  greater,  indicating  a  larger 
drop  in  voltage  at  full  load,  it  is  apparent  that  the  numerically 
greater  value  of  regulation  indicates  a  certain  inferiority  in  the 
construction  of  the  machine. 

Current  Supply. — From  the  Shunt  Generator  assigned. 

Apparatus  Required. —  (1)  Shunt  generator  driven  by  a  vari- 
able speed  motor;  (2)  field  rheostat;  (3)  speed  indicator;  (4) 
double-pole  single-throw  switch;  (5)  lamp  bank  to  be  used  as  a 
load;  (6)  an  ammeter;  and  (7)  a  voltmeter. 

Order  of  Work.— 1.  Connect  the  field  rheostat  between  the  field 
winding  and  the  armature  terminals  for  shunt  (self  excitation) 
operation.  Drive  the  generator  at  its  normal  speed,  and  main- 
taining constant  speed,  observe  and  record  the  terminal  voltage 
at  no-load  for  5  different  positions  of  the  field  rheostat  handle, 
starting  with  all  the  resistance  cut  in  and  gradually  decreasing 


36 


LABORATORY  MANUAL 


the  resistance.  Use  Form  7.  (Note:  Although  not  shown  in  Fig. 
12,  it  will  be  an  advantage  to  connect  an  ammeter  in  the  field 
circuit  as  a  guide  to  the  changes  which  take  place  in  the  field 
current  in  item  1  as  the  field  rheostat  is  adjusted;  to  insure  in 
item  2  that  the  field  current  remains  constant ;  and  as  a  guide  to 
the  changes  in  the  field  current  in  items  3  and  4.) 


^ 

Constant  Speed 

Constant  Field  Current 

£ 

Constant  Speed 

Position  of 
Field  Rheostat 

Volts 

Speed 

Volts 

Output  Amperes 

Volts 

1 

1     (All  In) 

1 

(Zero  Load) 

2 

2 

2 

(Half  Load) 

3 

3 

3 

(Zero  Load) 

Form  7. 

2.  Adjust  the  terminal  voltage  by  means  of  the  field  rheostat 
for  its  normal  value  at  normal  speed  and,  leaving  the  field  rheo- 
stat untouched,  reduce  the  speed  to  a  value  20  per  cent,  below 
normal,  and  observe  and  record  the  terminal  voltage  at  no-load 
for  the  low  speed  and  for  4  other  values  of  speed,  gradually  in- 
creasing it  until  somewhat  above  normal. 


Field  Rheostat 


Ammeter 


66666 


Lamp  Bank  Used  as  Load 


Fig.  12. — Diagram  for  loading  a  shunt  generator.  The  lamps  may 
conveniently  be  disconnected  from  the  machine  by  the  main  switch.  A 
voltmeter  is  to  be  connected  to  the  armature  terminals. 

3.  Connect  the  lamp  bank  through  the  double-pole  single- 
throw^  switch  and  an  ammeter  to  the  armature  terminals  as  shown 
in  Fig.  12.  With  the  switch  open  and  with  normal  speed  adjust 
the  voltage  to  its  normal  value  by  means  of  the  field  rheostat, 
after  which  the  field  rheostat  is  to  be  left  untouched.  Turn  off 


DIRECT  CURRENT  37 

all  the  lamps,  close  the  switch,  and  then  turn  on  enough  lamps 
to  load  the  machine  to  about  50  per  cent,  of  its  capacity  (see 
name  plate  on  the  machine).  Keeping  the  speed  at  its  normal 
value  throughout,  observe  and  record  the  terminal  volts  before 
and  after  throwing  on  the  lamps.  Use  Form  7. 

4.  Same  as  3  except  that  full  load  current  is  to  be  used. 

Written  Report. — 1.  In  item  1,  Order  of  Work,  why  do  the 
changes  of  the  field  rheostat  change  the  terminal  voltage  ?  What 
is  the  range  in  the  control  of  voltage  by  this  means  as  observed? 

2.  Same,  for  speed  change  in  item  2,  Order  of  Work. 

3.  In  item  3,  Order  of  Work,  what  causes  the  voltage  to  drop 
as  the  load  is  thrown  on  the  machine?     How  does  this  drop  in 
voltage  for  half  and  for  full  load  compare  ? 

4.  From  the  observations  under  item  4,  Order  of  Work,  cal- 
culate the  percentage  regulation  of  the  machine. 

5.  From  the  observations  in  this  experiment,  what  are  your 
conclusions  as  to  the  adaptability  of  the  shunt  generator  for  elec- 
tric lighting? 

EXPERIMENT  10. 

Shunt  and  Separate  Field  Excitation  Compared. 
See  the  Theory  under  Experiment  8  in  the  Manual. 

The  object  of  this  experiment  is  to  afford  an  opportunity  for  a 
study  of  the  factors  entering  into  the  changes  of  voltage  due  to 
loading  a  shunt  wound  generator  both  for  shunt  (self)  and  sepa- 
rate field  excitation. 

Theory. — In  the  shunt  wound  generator  connected  for  self  ex- 
citation (shown  in  Fig.  10),  as  the  load  is  increased  the  volts 
(RI)  drop  in  the  armature  increases  and,  hence,  the  terminal 
voltage  decreases.  With  each  decrease  in  terminal  voltage  the 
shunt  field  current,  equal  to  E/R,  falls  off,  and  as  a  consequence 
the  field  magnetism  and  the  induced  armature  voltage  in  turn 
are  reduced.  Hence,  a  decrease  in  terminal  voltage  produces 
what  may  be  termed  a  cumulative  reduction  of  terminal  voltage 
in  the  shunt  self-excited  machine. 

In  the  shunt  wound  generator  connected  for  separate  excita- 
tion (shown  in  Fig.  10),  as  the  load  is  increased,  the  volts  (RI) 
drop  in  the  armature  increases  and,  hence,  the  terminal  voltage 


38 


LABORATORY  MANUAL 


decreases,  however,  with  the  following  difference:  The  field  cur- 
rent in  this  case  is  independent  of  the  terminal  voltage  of  the 
armature,  being  connected  to  the  constant  voltage  supply  mains, 
and,  hence,  any  decrease  in  the  terminal  voltage  of  the  armature 
has  no  effect  on  the  field  current.  For  this  reason,  the  decrease 
in  terminal  voltage  is  less  for  a  given  load  current  in  the  case  of 
separate  than  for  shunt  (or  self)  excitation. 

Current  Supply. — 110  volts  Direct  Current. ' 

Apparatus  Required. —  (1)    Shunt  generator;    (2)    double-pole 
double-throw  switch   for  readily  connecting  the  field  winding 


able'  |-42_2H 

T.  r 


00000 


Lamp  Bank  Used  us  Load 


Field  Rheostat 


Fig.  13. — A  study  of  self  and  separate  excitation.  The  double-pole 
double-throw  switch  is  a  convenient  means  for  quickly  connecting  the 
field  winding  either  to  the  armature  terminals  or  the  supply  mains. 

either  to  the  supply  mains  or  the  armature  terminals ;  (3)  double- 
pole  single-throw  switch  for  connecting  the  armature  terminals 
to  the  load  of  lamps ;  (4)  field  rheostat ;  (5)  speed  indicator ;  (6) 
lamp  bank  to  be  used  as  the  load;  (7)  ammeter  for  the  field  cir- 
cuit; (8)  ammeter  for  the  main  circuit;  and  (9)  voltmeter. 

Order  of  Work. — 1.  Connect  the  field  winding  through  the 
double-pole  double-throw  switch  and  an  ammeter,  as  shown  in 
Fig.  13,  for  self  or  separate  excitation.  Connect  the  armature 
terminals  through  the  double-pole  single-throw  switch  and  an 
ammeter  to  the  lamp  bank.  "With  the  load  switch  open,  and  the 
field  winding  connected  to  the  armature  terminals,  drive  the 


DIRECT  CURRENT 


39 


machine  at  normal  speed  and  see  that  it  builds  up  to  normal 
voltage,  making  the  required  adjustments  by  means  of  the  field 
rheostat.  Throw  the  field  switch  for  separate  excitation  and  see 
that  the  instruments  read  in  the  same  direction  for  both  self  and 
separate  excitation.  If  not,  reverse  one  set  of  connections. 

2.  Throw  the  field  switch  for  self  excitation,  and  with  normal 
speed  and  normal  no-load  voltage,  throw  on  14  full  load  current, 
keeping  the  speed  constant  and  leaving  the  field  rheostat  un- 
touched after  the  preliminary  adjustment  for  normal  no-load 
voltage.  Observe  and  record  the  terminal  voltage,  field  current, 
load  current  and  speed  before  and  after  throwing  on  the  *4  load 
current.  Use  Form  8. 

(Speed  Constant  Throughout) 


JZ5 

Self  Excitation 

Separate  Excitation 

Volts 

Field  Amperes 

Load  Amperes 

Speed 

Volts 

Field  Amperes 

Load  Amperes 

Speed 

1 

(Zero  Load) 

2 

(Quarter  Load) 

3 

(Zero  Load) 

4 

(Half  Load) 

Form  8. 

3.  With  the  load  switch  open  throw  the  field  switch  for  sepa- 
rate excitation  and  repeat  the  observations  outlined  under  2. 

4.  Same  as  2  and  3,  except  that  %,  %  and  full  load  current 
are  to  be  used  in  turn. 

Written  Report. — 1.  Why  does  the  voltage  fall  off  more  for 
self  than  for  separate  excitation  in  items  2,  3,  and  4,  Order  of 
Work. 

2.  Plot  a  curve  using  volts  as  ordinates  and  load  current  as 
abscisses  for  zero,  %,  %,  %,  and  full  load  current,  both  for  self 
and  for  separate  excitation. 

3.  Why  do  the  values  of  field  current  change  in  the  case  of 
self  excitation,  but  not  for  separate  excitation   (assuming  con- 
stant voltage  supply  mains)  ? 

4.  Is  there  more  likelihood  of  a  self-excited  shunt  generator 
having  its  polarity  reversed  on  starting  up  than  a  separately  ex- 
cited machine ?    Why  ?    (See  Article  118  in  the  text  book.)     Give 


40  LABORATORY  MANUAL 

a  case  where  such  a  reversal  of  polarity  might  be  a  serious  disad- 
vantage, and  explain  briefly. 

5.  If  the  speed,  electromotive  force  and  current  to  a  lamp  bank 
are  the  same,  what  are  the  relative  field  currents  by  self  and  sepa- 
rate excitation  ? 

EXPERIMENT  11. 
Electrical  Features  of  the  Compound  Generator. 

See  Article  122  and  the  example  at  the  end  of  Article  122  in 
the  text  book,  also  the  Theory  under  Experiment  9  in  the  Manual. 

The  object  of  this  experiment  is  (a)  to  observe  the  tendency  of 
the  series  field  winding  in  a  compound  generator  to  offset  the  de- 
crease of  terminal  voltage  due  to  armature  volts  drop  (RI)  ;  (b) 
to  note  the  effect  on  the  terminal  voltage  produced  by  changing 
the  series  field  current  (by  means  of  a  shunt)  for  a  given  output 
current;  (c)  to  note  the  effect  on  the  compounding  by  a  change 
in  the  running  speed  of  the  machine,  the  initial  no-load  voltage 
being  the  same  as  in  (a)  ;  and  (d)  to  take  the  necessary  observa- 
tions on  the  generator  operated  as  a  shunt  machine  (the  series 
winding  disconnected)  for  a  determination  of  the  number  of 
series  turns  required  for  compounding. 

Theory. — Like  the  shunt  generator,  as  the  output  of  the  com- 
pound generator  increases,  the  terminal  voltage  tends  to  decrease 
due  to  the  volts  (RI)  drop  in  the  armature  winding.  The  series 
winding,  however,  on  the  compound  generator  through  which  all 
or  most  of  the  output  current  flows,  being  wound  on  the  field  mag- 
nets with  the  shunt  windings,  produces  a  magnetic  field  which  is 
nearly  proportional  to  the  load  current.  Hence,  when  the  arma- 
ture RI  drop  is  large,  that  is,  when  the  output  current  is  large, 
the  tendency  of  the  terminal  voltage  to  decrease  is  compensated 
for  by  the  additional  field  magnetism  produced  by  the  series 
winding,  which  causes  a  larger  voltage  to  be  induced  in  the  arma- 
ture. 

Obviously,  by  changing  the  number  of  series  turns  or  by  vary- 
ing the  proportion  of  the  full  load  current  which  flows  through 
the  series  winding,  the  degree  of  this  compensating  effect  may 
be  correspondingly  changed.  In  that  case  where  the  full  load 
and  no-load  voltages  are  the  same  in  value,  the  machine  is  said  to 


DIRECT  CURRENT  41 

be  flat  compounded.  Where  the  full  load  voltage  is  greater  than 
the  no-load  voltage,  the  machine  is  said  to  be  over-compounded. 

In  Experiment  9  it  was  shown  that  the  terminal  voltage  of  the 
shunt  generator  is  changed  by  variations  in  the  field  current.  As 
the  load  on  a  shunt  generator  increases,  the  field  current  might 
be  increased  sufficiently  for  each  increase  in  output  current  to 
maintain  a  constant  terminal  voltage  throughout  the  range  from 
no-load  to  full  load,  provided  there  was  enough  margin  in  the 
field  rheostat  resistance  to  permit  of  the  necessary  increases  in 
the  field  current.  As  shown  in  the  foregoing,  this  same  effect 
may  be  produced  by  means  of  a  series  winding  placed  on  the  mag- 
nets in  addition  to  the  shunt  winding,  the  necessary  increases  in 
field  magnetism  being  produced  by  the  current  output  which 
flows  through  the  series  turns. 

The  ampere-turn  (one  ampere  flowing  through  one  turn  of 
wire)  is  a  common  unit  of  magnetizing  effect.  Thus,  if  2  am- 
peres flow  through  5,000  turns  of  wire,  the  magnetizing  effect  is 
equivalent  to  1  ampere  through  10,000  turns.  In  the  latter  part 
of  this  experiment,  the  generator  is  operated  as  a  shunt  machine 
at  normal  voltage  and  speed  (speed  maintained  constant  through- 
out). Starting  with  no  load,  the  current  output  is  increased  in 
steps,  and  the  field  rheostat  is  adjusted  at  each  observation  for 
the  initial  terminal  voltage  value,  in  other  words,  the  voltage  is 
maintained  constant  throughout  by  the  hand  manipulation  of  the 
field  rheostat. 

As  an  illustration,  if  the  shunt  field  current  required  to  pro- 
duce the  initial  voltage,  at  full  load,  is  1.5  amperes,  while  at  no- 
load  the  field  current  required  is  1.0  ampere,  this  increase  of  0.5 
ampere  required  at  full  load,  multiplied  by  the  number  of  shunt 
field  turns  (assumed  as  10,000  this  product  is  0.5X10,000=5,000 
ampere- turns)  represents  the  additional  field  excitation  required 
at  full  load  in  order  to  maintain  constant  terminal  voltage. 

By  the  aid  of  a  series  winding  through  which  the  full  load  cur- 
rent flows  (assumed  to  be  100  amperes)  this  same  magnetizing 
effect,  that  is,  the  additional  5,000  ampere-turns,  may  be  secured 
by  the  use  of  50  series  turns.  Hence,  the  effect  at  full  load  is 
the  same  whether  0.5  ampere  is  added  to  the  no-load  value  of 
shunt  field  current  through  10,000  shunt  turns,  or  whether  the 
full  load  current  of  100  amperes  flows  through  50  turns  wound 
on  the  magnets  as  a  series  winding  in  addition  to  the  shunt  wind- 


42 


LABORATORY  MANUAL 


ing.     Obviously  the  series  turns  produce  no  effect  on  the  excita- 
tion at  no-load  if  connected  as  shown  in  Fig.  14. 

Current  Supply. — From  the  Compound  Generator  assigned. 

Apparatus  Kequired. —  (1)  Compound  generator  driven  by  a 
variable  speed  motor;  (2)  field  rheostat;  (3)  speed  indicator; 
(4)  double-pole  single-throw  switch;  (5)  low  resistance  to  be  used 


Voltmeter 


Shunt  Field 
Rheostat 


Ammeter 


Series  Field  '  <j 


00000 


Lamp  Bank  Used  as  Load 

Fig.  14. — Compound  generator,  using  the  short  shunt  connection. 
Note  that  no  current  flows  through  the  series  winding  when  the  main 
switch  is  open.  (Note:  This  diagram  shows  the  general  scheme  of 
connections.  As  indicated  by  the  windings,  the  machine  would  be 
differentially  compounded,  while  in  practice  it  is  ordinarily  arranged 
for  cumulative  compounding,  that  is,  the  series  winding  would  be  the 
reverse  of  that  shown.) 

as  a  shunt  around  the  series  field;  (6)  lamp  bank  to  be  used  as 
a  load;  (7)  two  ammeters;  and  (8)  a  voltmeter. 

Order  of  Work. — 1.  With  the  generator  arranged  for  compound 
operation,  connect  the  lamp  bank  through  the  double-pole  single- 
throw  switch  and  an  ammeter  to  the  machine  terminals,  as  shown 
in  Fig.  14.  With  the  switch  open  and  with  normal  speed,  adjust 
the  voltage  to  its  normal  value  by  means  of  the  shunt  field  rheo- 
stat. Turn  off  all  the  lamps,  close  the  switch,  and  then  turn  on 
enough  lamps  to  load  the  machine  to  about  50  per  cent,  of  its  ca- 
pacity (see  name  plate  on  the  machine).  Keeping  the  speed  at 
its  normal  value  throughout,  record  the  terminal  volts  and  load 
current  before  and  after  throwing  on  the  lamps.  The  field  rheo- 
stat is  to  be  untouched  after  the  initial  adjustment. 

2.  Same  as  1,  except  that  full  load  current  is  to  be  used. 


DIRECT  CURRENT 


43 


3.  Same  as  2,  except  that  the  low  resistance  shunt  is  to  be  con- 
nected to  the  terminals  of  the  series  winding,  and  its  resistance 
varied  until  the  terminal  voltage  is  5  per  cent,  lower  at  full  load 
than  in  the  observations  of  item  2. 

4.  Same  as  1  and  2,  except  that  the  machine  is  to  be  run  10 
per  cent,  below  rated  speed  throughout,  and  the  shunt  current 
adjusted  to  give  normal  no-load  voltage,  that  is,  the  same  value 
as  used  in  items  1  and  2.     (Note :  Due  care  must  be  taken  to  in- 
dicate on  the  data  sheet,  to  which  speed  the  observations  of  items 
1,  2  and  4  refer,  in  each  case.) 

5.  The  generator  assigned  is  to  be  arranged  for  shunt  opera- 
tion, that  is,  the  series  winding  is  to  be. disconnected,  and  the 
machine  is  to  be  connected  to  the  lamp  bank  as  shown  in  that 


£ 

Terminal  Volts 
(Constant) 

Field  Amperes 

Output  Amperes 

Speed  (Constant) 

1 

(Zero  Load) 

2 

(Quarter  Load) 

3 

(Half  Load) 

Form  9. 

portion  of  Fig.  13  involving  self  excitation,  that  is,  omit  the  two 
terminals  on  the  left  of  the  field  switch  shown  in  this  illustration. 

6.  Starting  at  no  load  (load  switch  open)  adjust  the  voltage  to 
its  normal  value  at  normal  speed  and  observe  the  terminal  volts, 
field  current,  output  current  (=zero)  and  speed.    Use  Form  9. 

7.  Turn  off  all  the  lamps,  close  the  switch,  and  then  turn  on 
enough  lamps  to  equal  ^  of  the  rated  capacity  of  the  machine, 
adjusting  the  field  rheostat  until  the  terminal  voltage  is  the 
same  as  in  6  and  keeping  the  speed  constant.     Observe  the  ter- 
minal volts,  field  current,  output  current  and  speed  as  in  Form 
9. 

8.  Same  as  7,  for  %,  %,  full  load  and  y±  overload  in  turn, 
maintaining  the  speed  constant  and  adjusting  the  field  rheostat 
for  constant  terminal  voltage  in  each  case. 

9.  Ascertain  and  record  the  number  of  shunt  field  turns  on 
the  generator. 


44  LABORATORY  MANUAL 

Written  Report. — 1.  In  item  1,  Order  of  Work,  what  causes  the 
voltage  to  be  maintained,  notwithstanding  the  effect  of  RI  drop 
in  the  armature?  How  does  the  terminal  voltage,  under  load, 
compare  in  items  1  and  2,  Order  of  Work,  for  half  and  full  load 
current  output  ? 

2.  What  effect  does  the  shunt,  around  the  series  field  terminals, 
have  on  the  terminal  voltage  for  a  given  load  output  ? 

3.  What  would  the  effect  be  of  increasing  the  number  of  series 
field  turns,  on  the  full  load  voltage  ?    On  the  no-load  voltage  ? 

4.  When  the  compound  generator  is  run  10  per  cent,  below 
normal  speed  in  item  4,  Order  of  Work,  with  the  shunt  current 
adjusted  to  give  normal  no-load  voltage,  how  does  the  compound- 
ing compare  with  that  at  normal  speed  ?    Explain. 

5.  Plot  a  curve  using  the  field  current  as  ordinates  and  out- 
put current  as  abscisses,  from  the  observations  in  items  6,  7  and 
8,  Order  of  Work. 

6.  Calculate  the  number  of  series  turns  necessary  for  flat  com- 
pounding. 

7.  If  the  speed  had  fallen  off  10  per  cent,  between  a  set  of  ob- 
servations, what  effect  would  this  have  had  on  the  necessary  in- 
crease of  field  current  to  bring  the  voltage  up  to  normal  value 
before  taking  the  next  set  of  observations  in  items  6,  7  and  8, 
Order  of  Work? 

8.  If  it  was  desired  to  calculate  the  series  turns  necessary  to 
over-compound  this  machine  by  5  per  cent,  at  full  load,  what  ad- 
ditional observations  would  have  been  necessary  in  the  experi- 
ment in  items  6,  7  and  8,  Order  of  Work? 

9.  Why  may  it  be  an  advantage  to  have  the  terminal  voltage 
of  a  compound  generator  higher  at  full  load  than  at  no-load  in 
some  cases  ?    Explain, 

10.  From  the  observations  in  this  experiment,  what  are  your 
conclusions  as  to  the  adaptability  of  the  compound  generator  for 
electric   lighting  and  power  service?     Are   generators   usually 
shunt  or  compound  ? 

EXPERIMENT  12. 
Study  of  the  Storage  Battery. 

The  object  of  this  experiment  is  to  gain  a  working  knowledge 
of  the  practical  construction  and  operation  of  the  storage  bat- 
tery. While  a  knowledge  of  the  principles  of  construction  and  of 


DIRECT  CURRENT  45 

the  theory  of  operation  is  an  advantage,  in  this  case  it  is  possibly 
more  important  to  understand  something  of  the  practical  opera- 
tion and,  hence,  this  feature  will  be  emphasized  somewhat  to  the 
exclusion  of  the  more  theoretical  items. 

Theory. — The  storage  cell  is  commonly  known  as  a  secondary 
cell  on  account  of  the  necessity  of  charging  before  taking  current 
from  a  battery  made  up  of  a  number  of  such  cells.  The  term 
secondary  further  distinguishes  the  storage  battery  from  the  pri- 
mary cell  where  the  electromotive  force  and  current  are  the  re- 
sult of  direct  chemical  action  on  the  elements  which  make  up  the 
battery. 

There  are  certain  practical  points  regarding  the  operation  of 
storage  batteries  which  are  referred  to  in  the  text  book,  but  the 
student  should  send  a  post  card  to  the  manufacturer  of  the  bat- 
tery examined,  with  a  request  for  the  circular  dealing  with  the 
operation  of  the  given  battery.  It  is  suggested  that  this  circu- 
lar be  attached  to  the  Written  Report  for  future  reference. 

Apparatus  Required. —  (1)  Several  grids  from  a  storage  bat- 
tery; (2)  a  regular  storage  battery  equipment  (found  in  most 
laboratories);  (3)  foot  rule;  (4)  voltmeter;  and  (5)  a  hydro- 
meter for  measuring  the  specific  gravity  of  the  electrolyte. 

Order  of  Work. — 1.  Make  an  inspection  of  the  storage  battery 
grids  available,  sketching  same  with  dimensions.  Describe  briefly 
the  construction  of  these  plates  on  the  data  sheet. 

2.  Inspect  the  storage  battery  equipment  in  the  Laboratory, 
measuring  approximately  the  size  of  the  grids  and  the  size  of  the 
glass  jars.     Note  the  height  of  the  electrolyte  in  the  jars,  and 
the  general  method  of  supporting  the  jars  and  of  making  connec- 
tions between  the  cells.    Make  sketches  or  describe  these  various 
items  briefly  on  the  data  sheet. 

3.  Measure  the  voltage  of  one  or  two  of  the  cells  separately 
and  of  the  entire  battery,  recording  these  observations. 

4.  Measure  the  voltage  of  the  battery  when  delivering  various 
currents  and  when  receiving  various  currents. 

5.  Count  and  record  the  number  of  cells  in  the  battery,  and 
measure  the  temperature  and  the  density  of  the  electrolyte. 

6.  Secure  the  name  and  address  of  the  manufacturer  of  the 
battery  examined. 


46  LABORATORY  MANUAL 

Written  Report.— 1.  What  is  the  active  material  in  the  grids 
inspected  in  item  1,  Order  of  Work?  How  is  this  active  material 
deposited  and  held  on  the  plates  in  the  manufacture  of  the  bat- 
tery ?  Of  what  metal  are  the  supporting  plates  constructed  ? 

2.  How  are  the  jars  of  the  battery  mounted,  and  what  precau- 
tion is  taken  in  the  battery  room  in  case  of  leakage  ? 

3.  What  is  the  average  electromotive  force  per  cell  of  the  bat- 
tery inspected?     What  is  the  average  rate  of  discharge  in  am- 
peres allowable  based  on  an  8-hour  rate?     (See  Article  200  in 
text  book.) 

4.  What  is  the  effect  on  the  battery  terminal  voltage  when  de- 
livering various  currents  ? 

5.  Same,  when  receiving  various  currents? 

6.  What  is  the  general  procedure  in  charging  a  battery  ? 

7.  What  precautions  must  be  observed  when  a  battery  is  to  be 
unused  for  several  months? 

8.  How  can  it  be  determined  when  a  battery  requires  charging 
and  when  it  is  sufficiently  charged? 

9.  What  results  if  a  battery  is  discharged  or  charged  at  too 
rapid  a  rate  ? 

(It  is  suggested  that  items  6,  7,  8  and  9,  of  Written  Report, 
be  answered  by  the  aid  of  the  pamphlet  secured  by  the  student 
from  the  manufacturer  of  some  standard  battery  equipment,  pre- 
ferably the  one  inspected.) 


EXPERIMENT  13. 
Study  of  the  Wiring  in  the  Laboratory. 

In  this  Experiment  the  opportunity  is  given  for  gaining  in- 
formation in  regard  to  methods  of  distributing  the  electric  cur- 
rent for  lighting  and  power  throughout  a  given  building.  The 
general  items  connected  with  such  work  are  here  given  preference 
over  details  and  such  few  details  as  are  desired  will  be  mentioned 
in  the  following  directions  under  appropriate  heads. 

Theory. — Interior  wiring  for  the  supply  of  electric  lamps  is  ar- 
ranged for  by  the  constant  voltage  system  of  distribution.  This 
means  that  each  lamp  must  receive  approximately  the  same  volt- 
age no  matter  how  many  are  turned  on  and,  hence,  the  size  of 


DIRECT  CURRENT  47 

wires  must  be  large  enough  in  central  portions  of  the  building 
where  heavy  currents  flow,  as  to  cause  only  a  low  voltage  (RI) 
drop,  while  in  more  remote  portions  of  the  building  the  size  of 
wires  may  be  smaller.  Power  mains  must  also  follow  this  general 
scheme. 

An  ordinary  method  used  for  lighting  circuits  is  to  run  heavy 
wires  from  the  main  switch  (or  bus  bars)  to  panel  (switch)  boxes 
on  the  various  floors,  and  from  these  panel  boxes  individual  cir- 
cuits of  660  watts  maximum,  are  run  to  the  various  rooms.  The 
660  watts  limit  is  fixed  by  the  National  Board  of  Fire  Under- 
writers as  a  safeguard  against  fire  risk.  Thus,  if  100  watt  tung- 
sten lamps  are  used,  each  circuit  is  limited  to  6  lamps. 

Order  of  Work. — 1.  In  the  room  assigned,  observe  and  record 
the  number,  size  and  type  of  lamps  and  reflectors  used ;  the  area 
of  the  floor  space  in  square  feet;  the  mounting  height  and  spac- 
ing of  the  lamps ;  and  the  arrangements  for  other  uses  of  current, 

2.  Kecord  by  sketches  and  explanation  the  method  of  mount- 
ing the  lamps  at  the  ceiling ;  the  switching  arrangement ;  and  the 
number  of  lamps  per  switch. 

3.  Note  the  general  arrangement  of  conduit  or  wiring  to  this 
room  from  the  panel  box,  and  inspect  the  panel  box,  making  a 
sketch  and  giving  dimensions  on  the  drawing;  note  the  fuses  and 
circuit  breakers;  also  the  kind  of  insulation  and  the  size  of  the 
wire  used. 

4.  Note  how  the  conduit  is  run  to  the  panel  box  from  the  main 
switch  board,  recording  such  data  as  may  be  necessary  for  an  ex- 
planation in  the  Written  Report. 

5.  Note  and  record  the  number  of  switches  on  the  main  switch 
board  which  supply  lighting  and  service  outlets  throughout  the 
building. 

6.  From  plans  or  measurements  determine  and  record  the  ap- 
proximate number  of  square  feet  area  of  the  floors  in  the  build- 
ing. 

Written  Report. — 1.  How  many  watts  per  square  foot  are  used 
for  lighting  in  the  room  assigned?  What  is  the  ratio  of  spacing 
to  mounting  height  of  the  lamps,  and  what  effect  does  this  have 
on  the  distribution  of  the  light  throughout  such  a  room  ? 


48,  LABORATORY  MANUAL 

2.  Describe  briefly  the  general  scheme  used  in  running  the 
wires  from  the  main  switch  board  to  the  lamps  in  the  building 
inspected. 

3.  Same,  for  the  service  outlets. 

4.  Based  on  the  watts  per  square  foot  used  for  lighting  in  the 
room  inspected,  and  on  the  total  floor  area  in  the  building,  how 
many  watts  are  required  as  a  maximum  for  lighting  the  Labora- 
tory?   Express  this  result  also  in  kilowatts. 

5.  Does  the  generator  which  supplies  the  lamps  need  to  have  a 
capacity   equal   to  this  maximum  requirement?      (Suggestion: 
Give  due  weight  in  answering  this  question  to  the  average  num- 
ber of  lamps  which  may  be  used  at  any  one  time  in  such  a  build- 
ing.)    What  size  of  generator  would  you  consider  about  proper 
for  such  a  lighting  load? 


EXPERIMENT  14. 
Shunt  Motor  Speed  Features. 

See  Articles  129,  132,  133,  135,  139  and  145  in  the  text  book, 
also  the  Theory  under  Experiments  4  and  6  in  the  Manual. 

The  object  of  this  experiment  is  (a)  to  make  a  study  of  the 
factors  by  which  the  speed  of  the  shunt  motor  may  be  varied  or 
controlled;  (b)  to  observe  the  tendency  of  the  speed  to  decrease 
with  increasing  load;  and  (c)  to  take  the  observations  for  the 
calculation  of  the  so-called  regulation  of  the  machine. 

Theory. — By  the  term  speed  control  is  meant  the  changing  of 
conditions  exterior  to  the  motor  for  the  purpose  of  obtaining  de- 
sired speed  values.  Thus,  changing  the  field  rheostat  (by  hand), 
the  speed  of  the  machine  may  be  varied  over  quite  a  wide  range ; 
or  changing  the  supply  voltage  across  the  armature  terminals 
(by  means  of  a  rheostat  in  series  with  the  armature  or  other- 
wise), the  speed  may  be  varied. 

On  the  other  hand,  as  the  load  supplied  by  the  motor  is  in- 
creased, for  example,  when  an  added  load  is  placed  on  a  hoist  be- 
ing raised  by  the  machine,  the  motor  slows  down  and  thus  per- 
mits an  increased  current  to  flow  into  the  armature  necessary  for 
developing  the  additional  mechanical  force  (or  torque).  The 


DIRECT  CURRENT 


49 


field  winding  being  connected  to  constant  voltage  supply  mains, 
receives  the  same  current  as  before  in  this  case.  This  drop  in 
speed  depends  on  inherent  properties  of  the  motor,  chief  of 
which  is  the  resistance  of  the  armature  winding.  Changes  which 
occur  in  the  speed  due  to  inherent  properties,  are  referred  to  as 
speed  regulation  to  distinguish  them  from  changes  which  are 
made  by  varying  conditions  exterior  to  the  machine,  and  referred 
to  as  speed  control. 

Among  the  principal  means  for  controlling  the  speed  of  a 


Supply  Mains  (110  Volts  D  C.) 

fl 

Starting 


Fig.  15. — Shunt  motor.  The  "adjustable  resistance"  is  not  ordinarily 
required  in  addition  to  the  starting  box,  but  is  here  used  merely  for 
convenience  in  varying  the  voltage  across  the  armature  terminals. 

shunt  motor  is  by  the  hand  manipulation  of  the  field  rheostat  re- 
sistance; and  that  by  changing  the  voltage  across  the  armature 
terminals  of  the  machine  by  a  rheostat  in  series  with  the  arma- 
ture only. 

The  main  items  which  govern  inherent  changes  in  the  sp>eed 
(that  is,  the  regulation)  are  the  resistance  of  the  armature  wind- 
ing together  with  slight  magnetic  reactions  in  the  armature  which 
tend  to  modify  the  effective  magnetic  field  produced  by  the  field 
winding. 

The  percentage  regulation  of  the  speed  is  defined  as  the  differ- 
ence between  full  load  and  no-load  speed  divided  by  the  full 
load  speed,  supply  voltage  and  field  current  remaining  un- 
changed. ( Obviously  this  result  must  be  multiplied  by  100  to  ex- 


50 


LABORATORY  MANUAL 


press  it  as  a  percentage.)  Thus,  if  the  full  load  and  no-load 
speeds  are  1,500  and  1,650  revolutions  per  minute  respectively, 
the  regulation  is  equal  to  10  per  cent.  If  the  speed  at  full  load 
falls  off  more  than  indicated  in  this  case,  the  numerical  value  of 
the  regulation  will  be  greater  and,  hence,  will  indicate  a  certain 
inferiority  in  the  construction  of  the  machine. 

Current  Supply. — 110  or  220  volts  Direct  Current. 

Apparatus  Required. —  (1)  Shunt  motor;  (2)  starting  box;  (3) 
field  rheostat;  (4) 'armature  rheostat ;  (5)  speed  indicator;  (6) 
brake  to  be  used  for  loading  the  motor;  (7)  ammeter  for  the  field 
circuit;  (8)  ammeter  for  the  armature  circuit;  and  (9)  volt- 
meter. 


No. 

Position  of 
Field  Rheostat 

Speed 

Armature 
Amperes 

Position  of 
Armature  Rheostat 

Speed 

Armature 
Amperes 

Armature  Amperes 

Speed 

1 

1  (All  Out) 

(For  No 
Load) 

(Out) 

(Zero 
Load) 

(Zero  Load) 

2 

2 

1 

(In) 

• 

(Quarter  Load) 

3 

3 

« 

(Out) 

(Full 
Load) 

(Half  Load) 

4 

4 

• 

(In) 

- 

Form  10. 

Order  of  Work. — 1.  Connect  the  motor  to  the  supply  mains  as 
shown  in  Fig.  15.  With  the  field  rheostat  all  cut  out,  start  the 
motor  by  means  of  the  starting  box  and  cut  out  all  the  armature 
rheostat  resistance.  With  the  motor  unloaded,  observe  and  re- 
cord the  speed  for  this  and  for  4  other  positions  of  the  field 
rheostat,  gradually  increasing  its  resistance  until  the  motor  speed 
is  considerably  above  normal.  Use  Form  10. 

2.  With  the  motor  unloaded,  adjust  the  field  rheostat  for  nor- 
mal speed,  and  observe  the  speed  for  no  resistance  and  for  a 
fairly  high  resistance  inserted  in  the  armature  circuit,  leaving 
the  field  rheostat  untouched  after  the  initial  adjustment. 

3.  Same  as  2  except  that  the  motor  is  to  be  loaded  by  the  brake 
until  the  armature  current  is  a  fair  proportion  of  the  full  rated 
current.     (Note :  To  be  most  instructive  the  resistance  inserted  in 
the  armature  circuit  for  the  second  observation  in  items  2  and  3 
should  be  the  same  value  in  each  case.) 


DIRECT  CURRENT  51 

4.  Adjust  the  motor  for  normal  speed  at  no  load  by  the  field 
rheostat  after  reducing  the  armature  rheostat  resistance  to  zero, 
and  observe  and  record  the  speed  for  zero,  %,  %,  %  and  full 
load  armature  currents  in  turn  (assume  full  load  armature  cur- 
rent as  equal  to  the  current  rating  on  the  name  plate  of  the  ma- 
chine). Leave  the  field  rheostat  untouched  throughout  these 
observations  after  the  initial  adjustment. 

Written  Report. — 1.  In  item  1,  Order  of  Work,  why  does  the 
speed  increase  as  the  field  rheostat  resistance  is  increased  ?  "What 
is  the  range  of  speed  control  by  means  of  the  field  rheostat 
method  as  observed  ? 

2.  In  items  2  and  3,  Order  of  Work,  why  is  the  speed  affected 
more  by  the  armature  rheostat  when  the  motor  is  loaded  than 
when  unloaded? 

3.  From  the  observations  in  item  4,  Order  of  Work,  plot  a 
curve  using  speed  as  ordinates  and  armature  current  as  abscissas. 

4.  Calculate  the  percentage  speed  regulation  of  the  motor. 

5.  The  torque  of  a  motor  depends  on  the  field  magnetism  ef- 
fective in  the  armature  and  on  the  armature  current.    When  the 
field  magnetism  was  weakened  in  item  1,  Order  of  Work,  the 
motor  speeded  up.    Should  not  a  weakened  field  cause  less  torque 
and,  hence,  less  rather  than  more  speed?     Explain.      (See  Ar- 
ticle 135  in  the  text  book.) 

6.  To  what  is  the  decrease  in  speed  with  increased  load  due  as 
observed  in  item  4,  Order  of  Work? 


EXPERIMENT  15. 
Efficiency  of  a  Shunt  Motor  by  the  Brake  Method. 

See  Article  158  in  the  text  book. 

The  object  of  this  experiment  is  to  determine  the  efficiency 
(output  divided  by  input)  of  a  shunt  motor  by  measuring  the 
mechanical  output  and  the  electrical  input  at  various  loads. 

Theory. — The  efficiency  of  the  electric  motor  like  that  of  other 
machines  is  defined  as  the  ratio  of  output  to  input.  In  a  deter- 
mination of  the  efficiency,  therefore,  it  is  necessary  to  measure 
the  mechanical  output  as  indicated  by  a  brake  attached  to  the 


52  LABORATORY  MANUAL 

pulley,  and  for  each  value  of  output  thus  observed  to  measure 
the  electrical  input. 

In  the  actual  calculation  of  the  efficiency,  the  mechanical  out- 
put in  horse-power  may  readily  be  transformed  by  multiplying 
the  horse-power  by  746,  the  number  of  watts  in  one  horse-power, 
and  the  output  is  thus  expressed  in  the  same  units  as  the  input. 

Since  the  losses  in  a  motor  are  partly  constant  (or  nearly  con- 
stant) and  partly  variable,  and  since  the  variable  losses  (RP)  in 
the  armature  winding  vary  as  the  square  of  the  current,  it  will 
be  obvious  first,  that  the  constant  losses  play  a  much  larger  part 
for  low  loads,  thus  reducing  the  efficiency  at  low  loads  as  com- 
pared to  full  load,  and  second,  that  the  RP  loss  in  the  armature, 
increasing  as  the  square  of  the  current,  becomes  quite  large  in 
proportion  for  high  current  values  and,  hence,  the  efficiency  tends 
to  fall  off  after  a  certain  maximum  value  has  been  reached  near 
full  load. 

Current  Supply. — 110  or  220  volts  Direct  Current. 

Apparatus  Required. —  (1)  Shunt  motor;  (2)  starting  box;  (3) 
field  rheostat;  (4)  speed  indicator;  (5)  brake  to  be  used  for  load- 
ing the  motor;  (6)  ammeter;  and  (7)  voltmeter. 

Order  of  Work. — 1.  Connect  the  motor  to  the  supply  mains  in 
the  usual  manner,  arranging  the  ammeter  to  measure  the  entire 
current  input  to  both  field  and  armature. 

2.  Start  the  motor  and  bring  it  to  normal  speed  at  no  load,  that 
is,  with  the  brake  detached  completely  from  the  pulley.     Ob- 
serve and  record  the  input  (volts  and  amperes). 

3.  Attach  the  brake  and  tighten  until  the  ammeter  indicates 
%  the  full  rated  current  (see  name  plate  on  the  machine).    Ob- 
serve and  record  the  torque  exerted  at  the  pulley  and  the  volts 
and  amperes  input. 

4.  Same  as  3,  for  y2,  %,  full  load,  and  1*4  and,  if  practicable, 
P/2  of  full  load  current  in  turn. 

Written  Report. — 1.  Calculate  the  efficiency  of  the  motor  in  per 
cent,  from  the  observations  in  items  2,  3  and  4,  Order  of  Work. 

2.  Plot  a  curve,  using  the  efficiency  as  ordinates,  and  the  input 
current  as  abscissas. 


DIRECT  CURRENT  53 

3.  Explain  briefly  the  general  shape  of  this  curve,  that  is,  why 
it  follows  the  form  taken. 

4.  Name  the  constant  and  variable  losses  in  a  generator  or 
motor.     (See  Article  155  in  the  text  book.) 

5.  Why  should  the  constant  losses  in  a  motor  cause  the  effici- 
ency to  be  low  at  low  loads  ? 

6.  Why  does  the  RP  loss  in  the  armature  cause  a  reduction  of 
the  efficiency  after  a  certain  maximum  value  near  full  load  ? 

7.  How  could  the  armature  current  be  found  mathematically 
for  a  given  machine  at  which  the  efficiency  is  a  maximum  ? 


EXPERIMENT  16. 

Series  Motor  Speed  Features. 

See  Articles  140,  141,  142,  143,  144  and  145  in  the  text  book. 

The  object  of  this  experiment  is  to  make  a  study  of  the  speed 
of  a  series  motor  as  affected  by  the  load. 

Theory. — The  torque  of  a  series  motor  is  proportional  to  the 
field  magnetism  and  to  the  armature  current.  Since  the  field 
winding  and  the  armature  are  connected  in  series  and,  hence, 
the  same  current  flows  through  each,  the  torque  is  roughly  pro- 
portional to  the  square  of  the  armature  current. 

Under  a  light  load,  the  series  motor  takes  but  little  current  to 
produce  the  torque  required  and,  hence,  the  resistance  in  series 
with  the  motor  on  starting  is  made  large  enough  to  allow  only  a 
small  current  to  pass  through  the  motor.  The  motor  under 
these  conditions  speeds  up  rapidly  and  the  greater  the  speed  the 
more  counter  electromotive  force  induced  and,  hence,  the  less  the 
current  through  the  machine  until  the  resistance  is  cut  out.  As 
shown  with  the  shunt  motor  (See  Experiment  14),  to  weaken 
the  field  increases  the  speed  and,  hence,  the  speed  of  an  unloaded 
series  motor  becomes  excessive  and  would  damage  the  machine  if 
allowed  to  run  under  this  condition.  The  series  motor  must, 
therefore,  alwrays  be  connected  to  its  load,  as  in  street  cars  where 
the  motor  is  geared  or  mechanically  connected  to  the  load.  As  a 
precaution,  therefore,  always  see  that  a  load  is  connected  or 
coupled  to  the  series  motor  before  connecting  it  to  the  supply 
mains. 


54  LABORATORY  MANUAL 

Under  load,  the  operation  of  the  series  motor  is  somewhat  dif- 
ferent from  that  of  the  shunt  motor.  For  example,  if  the  load 
on  a  series  motor  be  doubled,  the  field  current  as  well  as  the  ar- 
mature current  is  increased,  so  that  the  speed  is  reduced  much 
more  than  in  the  shunt  motor  where  the  field  current  is  practi- 
cally constant  at  all  loads.  Again,  if  the  armature  current  be 
doubled  in  a  series  motor,  the  torque  is  increased  nearly  four 


Supply  Mains 

(110  Volts  D.  C.) 

A 

-M  •£    f  I— 

Ammeter 


Pig.  16. — Series  motor.  The  load,  which  must  be  connected  to  the 
motor  throughout  the  experiment,  is  not  shown  in  this  diagram.  The' 
adjustable  resistance  represents  the  starting  controller  used  with  series 
motors. 

times,  since  both  armature  and  field  current  are  doubled,  while 
to  double  the  armature  current  in  the  shunt  motor  merely  doubles 
the  torque  (the  shunt  field  current  remaining  constant). 

Hence,  under  heavy  loads,  due  to  the  fact  that  the  large  start- 
ing armature  current  flows  through  the  series  field  in  the  series 
motor,  it  has  a  greater  starting  torque  than  the  shunt  motor 
where  the  field  current  remains  sensibly  constant  irrespective  of 
the  value  of  the  armature  current. 

The  series  motor  is,  therefore,  well  adapted  to  those  cases  where 
a  large  starting  torque  is  desirable  and  where,  under  heavy  loads, 


DIRECT  CURRENT 


55 


the  speed  should  fall  in  order  that  the  power  requirements  and, 
hence,  the  current  may  not  be  excessive  as  in  street  car  opera- 
tion. Where  constant  speed  at  all  loads  is  a  necessary  require- 
ment, the  series  motor  is  not  adapted,  as  its  speed  variation  be- 
tween small  and  heavy  loads  is  very  large  as  compared  with  that 
of  the  shunt  motor.  Note  that  a  given  value  of  current  through 
the  series  motor  produces  the  same  torque  whatever  the  speed. 

Current  Supply. — 110  or  220  volts  Direct  Current. 

Apparatus  Required. —  (1)    Series  motor;    (2)    starting  resist- 
ance, (some  form  of  rheostat  having  a  fairly  large  current  carry- 


Lever  Arm  = 

No. 

Force  at  End 
of  Brake  Lever 

Amperes 

Volts 

Speed 

Shunt 

1 

Without  Shunt 
Around  Series  Winding 

f> 

3 

4 

5 

6 

"5  G 

^  "7) 

Form  11. 

ing  capacity);    (3)   speed  indicator;    (4)   brake  to  be  used  for 
loading  the  motor ;  (5)  ammeter;  and  (6)  voltmeter. 

Order  of  Work. — 1.  Connect  the  motor  to  the  supply  mains  as 
shown  in  Fig.  16,  and  arrange  to  have  the  brake  permanently  at- 
tached to  the  motor  pulley  throughout  the  experiment. 

2.  With  the  starting  resistance  all  in,  and  the  brake  mode- 
rately tight,  throw  in  the  main  switch  and  gradually  increase  the 
speed  by  cutting  out  the  starting  resistance.  Tighten  the  brake 
until  the  current  input  equals  %  more  than  the  current  rating 
on  the  name  plate  of  the  machine.  Observe  and  record  the 
torque  (the  tangential  force  at  the  rim  of  the  pulley  multiplied 
by  the  radius  of  the  pulley),  current,  volts  at  terminals  of  motor, 
and  speed.  Use  Form  11. 


56  LABORATORY  MANUAL 

3.  Reduce  the  load  until  the  input  current  equals  the  full  load 
rating  of  the  machine  and  repeat  the  observations  of  item  2. 

4.  Same  for  %,  %,  and  14  load  current  values  in  turn. 

5.  Place  a  low  resistance  around  the  series  field  terminals  as 
a  shunt.    Operate  the  machine  at  its  full  load  current  value  and 
observe  the  torque,  current,  volts  and  speed. 

Written  Report. — 1.  Explain  briefly  why  the  load  must  be  per- 
manently connected  to  a  series  motor. 

2.  Plot  a  curve  using  speed  as  ordinates  and  torque  as  ab- 
scissas from  the  observations  of  items  2,  3  and  4,  Order  of  Work. 

3.  Explain  the  decrease  in  speed  with  increasing  load  as  shown 
by  this  curve. 

4.  What  changes  were  observed  in  the  speed  and  torque  in 
item  5  as  compared  with  the  corresponding  observations  in  item 
3,  Order  of  Work?    Explain. 

5.  A  street  car  equipped  with  series  motors,  running  at  con- 
stant speed  on  the  level,  approaches  an  up  grade.     Explain  the 
action  of  the  motors  in  propelling  the  car  up  the  grade,  as  re- 
gards speed,  torque  and  current  in-take,  assuming  that  the  motor- 
man  leaves  the  controller  untouched. 

6.  Sometimes  on  climbing  a  steep  grade  a  motorman  throws 
the  controller  to  the  series  notch.    What  is  the  "series  notch", 
and  why  should  this  be  an  advantage  under  the  circumstances  ? 


EXPERIMENT  17. 
Efficiency ;  Stray  Power  Test ;  Brake  Test. 

See  Articles  155,  156  and  158  in  the  text  book,  also  the  Theory 
under  Experiment  15  in  the  Manual. 

Theory. — The  losses  in  a  generator  or  motor  may  be  classed 
under  the  head  either  of  losses  which  may  readily  be  calculated, 
or  of  losses  which  are  not  subject  to  calculation.  Thus  the  resist- 
ance losses  (RP)  may  easily  be  calculated  after  measuring  the 
resistance  of  field  and  armature,  and  from  the  currents  involved. 
The  friction  losses  in  the  bearings  and  at  the  brushes  of  the  ma- 
chine and  in  windage  and  the  losses  in  the  iron  of  the  machine 
(usually  called  hysteresis  and  eddy  current  losses)  cannot  easily 
be  calculated  and  are  referred  to  as  stray  power  loss.  That  is, 


DIRECT  CURRENT  57 

the  stray  power  loss  includes  all  the  losses  in  a  generator  or  motor 
except  the  RP  or  resistance  losses,  and  is  practically  constant 
independent  of  the  load. 

A  simple  experiment  to  determine  the  stray  power  loss  in  a 
generator  or  motor  is  to  drive  the  machine  as  an  unloaded  motor 
and  to  measure  the  power  input  under  this  condition.  Obviously 
this  input  is  all  loss,  since  there  is  no  useful  power  being  deliv- 
ered at  the  pulley.  If,  from  this  input  at  no  load,  the  RP  losses  in 
both  field  and  armature  be  subtracted,  the  remainder  represents 
the  stray  power  loss  in  watts. 

Inasmuch  as  the  stray  power  loss  is  sensibly  constant  at  all 
loads,  the  efficiency  of  a  generator,  for  example,  may  be  calcu- 
lated when  the  stray  power  loss  is  known  for  any  assumed  load 
by  the  use  of  the  equation  for  effiicency : 

Efficiency  =  OutP"L_ 

Output  -f  alJ  losses 

Output 


Output  -j-  stray  power  loss  -{-  RI2  losses 

Suppose,  for  example,  it  was  desired  to  calculate  the  efficiency 
at  half  load.  If  the  generator  is  rated  at  10  kilowatts,  and  the 
stray  power  loss  is  found  by  experiment  to  be  500  watts,  the  ef- 
ficiency may  be  calculated  by  a  substitution  in  the  equation  as 
follows : 

. 5000 

5000  +  500  +  RP  loss  in  field  and  armature 

The  RP  loss  in  the  field  and  armature  are  easily  calculated  from 
the  resistance  of  the  two  windings,  the  terminal  voltage  and  the 
armature  current  corresponding  to  the  assumed  load.  Thus  the 
RJ2  loss  in  the  field  is  equal  to  RlX(E/Rl)2,  and  the  RP  loss 
in  the  armature  is  the  resistance  of  the  winding  (R2)  multiplied 
by  the  square  of  half  the  full  load  current  as  indicated  on  the 
name  plate  of  the  machine. 

(Note :  While  the  stray  power  loss  varies  slightly  for  different 
loads  and  speeds,  it  is  treated  as  constant  in  this  experiment  for 
simplicity. ) 

Current  Supply.— 110  or  220  volts  Direct  Current. 

Apparatus  Required. — (1)  Shunt  machine;  (2)  starting  box; 
(3)  field  rheostat ;  (4)  speed  indicator ;  (5)  ammeter  for  the  field 


58  LABORATORY  MANUAL 

circuit;    (6)   ammeter  for  the  armature  circuit;  and   (7)   volt- 
meter. 

Order  of  Work.  —  1.  Connect  the  machine  to  the  supply  mains 
through  the  starting  box,  arranging  an  ammeter  in  both  field  and 
armature  circuits. 

2.  Run  the  machine  as  a  motor  at  normal  speed  and,  with  no 
load,  observe  and  record  the  amperes  to  both  field  and  armature 
and  the  volts  at  the  motor  terminals.     (Note  :   Since  the  starting 
current  of  an  unloaded  motor  is  apt  to  be  larger  than  its  nor- 
mal running  current  after  starting,  be  sure  to  close  the  short  cir- 
cuiting switch  about  the  armature  ammeter  before  starting  the 
motor.     This  makes  possible  the  use  of  an  instrument  of  low 
range  for  observing  the  rather  small  currents  of  the  motor  while 
in  operation  at  no  load.) 

3.  Record  the  full  rated  current  and  voltage  of  the  machine 
as  indicated  on  the  name  plate. 

4.  Shut  down  the  machine  and  measure  the  resistance  of  the 
armature  by  the  voltmeter-ammeter  method  as  described  in  Ex- 
periment 2. 

5.  Arrange  to  load  the  machine  by  a  brake,  and  observe  and 
record  the  torque,  speed,  current  in  field  and  armature,  and  ter- 
minal volts  at  %   %         and  full  rated  load  in  turn. 


Written  Report.  —  1.  From  the  observations  in  items  2  and  4, 
Order  of  Work,  calculate  the  stray  power  loss,  and  the  field  and 
armature  resistance  of  the  machine  used. 

2.  From  item  5,  Order  of  Work,  calculate  the  output  and  in- 
put to  the  motor  in  watts,  and     calculate  the  efficiency  of  the 
motor  by  the  brake  method  for  these  observations. 

3.  Calculate  the  efficiency  of  the  motor  for  14,  l/2,  %  and  full 
load  in  turn  by  the  stray  power  method,  using  the  stray  power 
loss  as  found  in  item  1,  Written  Report,  and  using  the  terminal 
volts  and  input  current  at  full  load  as  given  on  the  name  plate 
of  the  machine.     Compare  these  calculations  of  efficiency  with 
those  found  by  direct  measurement  in  item  2,  Written  Report. 

4.  Calculate  the  efficiency  of  the  machine  by  the  stray  power 
method  if  run  as  a  generator  at  full  load,  assuming  the  terminal 
volts  and  output  current  at  full  load  to  be  the  value  given  on  the 
name  plate  of  the  machine. 


DIRECT  CURRENT 


59 


EXPERIMENT  18. 
Static  Torque  Test  on  a  Motor. 

See  Article  125  in  the  text  book,  also  the  Theory  under  Ex- 
periment 4  in  the  Manual. 

The  object  of  this  experiment  is  to  make  a  study  of  the  torque 
of  a  motor  in  terms  of  the  field  and  armature  current,  while  the 
machine  is  at  rest. 


Supply  Mains  (110  Volts  D.  C.) 

-4 

A 

1  -S  f 

T— 

A 

Uii  t 

i  — 

Fig.  17. — Study  of  the  torque  produced  in  an  armature  for  various 
values  of  field  and  armature  currents.  Note  that  the  currents  in  field 
and  armature  can  be  adjusted  independently. 

Theory. — The  mechanical  force  which  turns  an  electric  motor 
is  produced  by  the  action  of  the  magnetic  field  on  the  current  in 
the  armature  conductors.  The  simplicity  of  the  elements  which 
produce  motion  in  the  motor  are  sometimes  lost  sight  of  on  ac- 
count of  the  conditions  which  determine  the  armature  current, 
such  as  load,  speed  and  the  like.  In  this  experiment,  these  sec- 
ondary conditions  are  eliminated  by  taking  the  observations  on 
the  motor  when  at  rest,  and  the  definite  relation  of  field  and 
armature  current  to  the  torque  produced  is  thus  emphasized. 

The  magnetic  field  is  not  directly  proportional  to  the  current 
which  produces  it,  because  as  the  field  magnets  become  saturated, 
the  magnetism  ceases  to  increase  in  direct  proportion  to  the  cur- 


60  LABORATORY  MANUAL 

rent  (see  Article  98  in  the  text  book).  Hence,  in  this  experi- 
ment, if  the  torque  is  not  found  to  vary  directly  with  the  field 
current  throughout  the  observations,  it  must  be  remembered  that 
the  torque  is  varying  directly  with  the  magnetic  field,  but  the 
field  is  not  varying  directly  with  the  field  current  due  to  satura- 
tion of  the  iron.  Obviously  the  saturation  effect  will  not  be  very 
noticeable  for  small  values  of  the  field  current. 

Current  Supply. — 110  or  220  volts  Direct  Current. 

Apparatus  Required. —  (1)  Shunt  motor;  (2)  resistances  for 
both  field  and  armature  circuits ;  (3)  Prony  brake;  (4)  ammeter 
for  field  circuit;  (5)  ammeter  for  armature  circuit;  and  (6) 
voltmeter. 

Order  of  Work. — 1.  Connect  the  field  winding  through  a  field 
rheostat,  which  should  possess  a  wide  range  of  adjustment,  to 
the  supply  mains ;  also,  the  armature  through  a  suitable  rheostat 
to  the  supply  mains;  each  to  have  its  own  switch  as  shown  in 
Fig.  17. 

2.  Clamp  the  brake  tightly  to  the  pulley  of  the  motor  and  with 
all  the  field  and  armature  resistance  cut  in,  throw  in  first  the 
field  arid  then  the  armature  switch.     See  that  the  torque  acts 
against  the  opposition  of  the  brake,  and  that  the  armature  does 
not  rotate. 

3.  With  a  constant  field  current,  that  is,  with  normal  volts 
across  the  field  terminals,  adjust  the  armature  current  to  %  full 
load  value  and  observe  and  record  the  field  and  armature  cur- 
rent and  the  torque  produced. 

4.  Same  as  3,  using  %,  %,  full  load  and  l1/^  load  currents 
through  the  armature  in  turn. 

5.  With,  say,  full  load  armature  current,  reduce  the  field  cur- 
rent to  %  its  normal  value,  and  observe  and  record  the  field  and 
armature  currents  and  the  torque  produced. 

6.  Keeping  the  armature  current  constant,  repeat  the  obser- 
vations of  item  5  for  %,  normal  and  l1^  normal  values  of  the 
field  current. 

7.  Maintaining  constant  field  current  and  constant  torque,  de- 
crease the  resistance  in  series  with  the  armature  and  allow  the 
motor  to  run.     Find  and  record  the  relation  between  armature 
current  and  torque  for  a  number  of  different  speeds;  also  the 
relation  between  the  speed  and  the  volts  across  the  armature 
terminals  for  each  of  these  values  of  speed. 


DIRECT  CURRENT  61 

Written  Report. — 1.  From  the  observations  in  items  3  and  4, 
Order  of  Work,  how  does  the  torque  (or  mechanical  force)  at 
the  pulley  vary  with  the  armature  current  for  a  constant  value 
of  field  current  f 

2.  From  items  5  and  6,  Order  of  Work,  how  does  the  torque 
vary  with  the  field  current  for  a  constant  value  of  armature  cur- 
rent? 

3.  If  the  torque  did  not  vary  directly  with  the  field  current  in 
the  experiment,  to  what  is  such  irregularity  due  ? 

4.  From  the  general  observations  of  this  experiment,  explain 
why  a  shunt  motor  must  slow  down  when  an  added  load  is  thrown 
on  its  pulley  if  it  is  to  carry  this  added  load  ? 

5.  From  item  7,  Order  of  Work,  what  is  the  relation  between 
armature  current  and  torque  at  different  speeds;  and  what  is 
the  relation  between  speed  and  volts  across  the  armature  in  each 
of  these  cases  ?    Explain  briefly. 


EXPERIMENT  19. 
Shunt  Generators  in  Parallel. 

The  object  of  this  experiment  is  to  observe  the  factors  which 
enter  into  the  operation  of  shunt  generators  in  parallel,  first,  as 
regards  the  necessary  conditions  for  throwing  one  generator  in 
parallel  with  another  machine,  and  second,  as  to  the  items  which 
are  involved  in  the  equal  or  proportionate  sharing  of  the  total 
output  of  a  power  station  by  the  various  generators  connected 
in  parallel  for  supplying  this  total  output. 

Theory. — In  many  electric  stations  it  is  the  practice  to  supply 
power  from  bus  bars  to  which  are  connected  a  number  of  genera- 
tors, each  delivering  its  share  of  the  total  load  supplied  from  the 
common  bus  bars.  In  this  way,  when  the  load  requirements  are 
low,  say  during  the  day  in  a  lighting  station,  a  few  of  the  gen- 
erators may  be  operated  at  or  near  full  load  and,  hence,  at  high 
efficiency,  and  as  the  total  output  of  the  station  increases,  one 
after  another  of  the  remaining  machines  may  be  connected  to  the 
bus  bars  in  parallel  with  those  already  in  operation.  Obviously 
the  positive  terminal  of  each  machine  must  be  connected  to  the 
positive  terminal  of  the  bus  bars. 


62  LABORATORY  MANUAL 

If  before  connecting  a  machine  to  the  bus  bars  its  voltage  is 
just  equal  to  that  of  the  bus  bars,  no  current  will  flow.  If  the 
voltage  induced  in  the  armature  be  slightly  higher,  a  current  will 
flow.  Suppose  the  machine,  when  carrying  no  load,  has  a  voltage 
3  per  cent,  higher  than  the  -bus  bars  and  that  before  it  is  con- 
nected to  the  bus  bars  it  is  loaded  until  the  terminal  electromo- 
tive force  decreases  (due  principally  to  the  RI  drop  in  the  arma- 
ture) and  becomes  equal  to  that  on  the  bus  bars.  If  now  the  load 
be  thrown  off  quickly  and  the  machine  be  connected  to  -the  bus 
bars,  it  is  in  condition  to  continue  delivering  the  same  current  to 
the  bus  bars  that  it  formerly  delivered  to  its  independent  load. 

Hence,  if  the  bus  bar  voltage  be  much  below  that  of  the  volt- 
age induced  in  the  armature  connected  to  it,  the  current  supplied 
by  the  armature  will  rise  when  the  two  are  connected,  until  the 
volts  (RI)  drop  in  its  armature  winding  and  the  leads  from  the 
armature  terminals  to  the  bus  bars  equals  the  difference  between 
the  bus  bar  and  the  induced  armature  voltage.  If  this  difference 
be  zero,  no  current  will  flow  from  the  machine;  while  if  the  dif- 
ference be  such  that  the  accompanying  RI  drop  in  the  leads  and 
armature  involves  a  current  greater  than  normal  for  the  machine, 
the  generator  will,  of  course,  be  overloaded. 

As  the  induced  voltage  of  a  generator  depends  on  the  field 
magnet  strength  for  constant  speed  conditions,  the  load  may  be 
increased  or  decreased  on  a  given  machine  connected  in  parallel 
with  others,  by  the  simple  variation  of  its  field  rheostat  resist- 
ance, assuming  that  the  driving  engine  delivers  a  corresponding 
increased  or  decreased  load. 

Where  two  similar  machines  of  the  same  capacity  are  arranged 
for  parallel  operation,  the  output  from  each  should  equal  one- 
half  of  the  total  power  supplied  by  the  bus  bars.  If  it  should 
be  desirable,  however,  to  reduce  the  load  on  one  of  the  machines 
and  yet  maintain  the  total  output  constant  at  a  constant  bus 
bar  voltage,  it  would  be  necessary  to  increase  the  field  resistance 
of  the  one  generator  to  lower  its  part  of  the  total  load,  and  to 
reduce  the  field  resistance  of  the  other  generator  to  increase  its 
part  of  the  total  load,  thus  maintaining  the  voltage  and  the  total 
output  at  a  constant  value.  In  this  way  the  total  load  may  be 
shifted  from  one  machine  to  another. 

Since  the  terminal  voltage  of  a  shunt  generator  varies  with  the 
load  (see  Experiment  9),  and  further,  since  this  change  of  volt- 


DIRECT  CURRENT 


age  with  load  is  not  apt  to  be  exactly  the  same  with  any  two  ma- 
chines, even  after  two  or  more  shunt  generators  are  adjusted  to 
give  their  share  of  the  total  load,  they  may  not  continue  to  share 
the  total  load  in  this  exact  proportion  for  all  bus  bar  or  total 
loads,  on  account  of  this  variation  in  the  voltage  changes  for  dif- 
ferent machines.  This  will  give  rise  to  slight  fluctuations  in  the 
sharing  of  the  loads,  which,  if  sufficiently  noticeable,  can  be  off- 
set by  hand  regulation  of  the  field  rheostats  when  necessary. 


Voltmeter 


Flexible  Leads 

J 

ri    <><)<)(j 

3 

j 

Lamp  Bank  Used  as  Load 

1 

Bus  Bars 

L 

1 

}— 

L 

A 

b  :§  i 

*— 

Ammeter 


Fig.  18. — Study  of  the  parallel  operation  of  shunt  generators.  A 
voltmeter,  not  shown  in  the  diagram,  is  to  be  available  for  measuring 
the  voltage  of  the  individual  machines. 

Current  Supply. — From  the  Shunt  Generators  assigned. 

Apparatus  Required. —  (1)  Two  shunt  generators;  (2)  lamp 
banks  to  be  used  as  a  common  load  supplied  from  bus  bars;  (3) 
field  rheostats  for  each  machine;  (4)  a  double-pole  single-throw 
switch  for  each  machine  and  for  the  total  load  (3  in  all)  ;  (5) 
three  ammeters,  one  for  each  machine  and  one  for  the  total  out- 
put current  from  the  bus  bars;  (6)  two  voltmeters,  one  for  the 
bus  bars  and  one  for  the  on-coming  machine. 


64  LABORATORY  MANUAL 

Order  of  Work. — 1.  Arrange  the  connections  of  the  two  assigned 
shunt  generators  as  shown  in  Fig.  18. 

2.  With  all  switches  open,  start  up  the  two  generators  and  ad- 
just the  voltage  of  each  to  its  normal  value. 

3.  Connect  one  of  the  generators  ("A")  to  the  bus  bars  (two 
lengths  of  wire)  and  turn  on  enough  lamps  to  load  the  machine 
to  its  full  capacity. 

4.  Adjust  the  voltage  of  the  other  generator   ("B")   to  the 
same  value  as  the  bus  bar  voltage  and  connect  it  to  the  bus  bars, 
being  sure  that  the  positive  terminal  of  the  machine  is  connected 
to  the  positive  terminal  of  the  bus  bars. 

5.  Vary  the  field  rheostat  of  machine  "B"  until  the  current 
in  the  two  machines  has  the  same  value,  at  the  same  time  adjust- 
ing the  field  rheostat  of  machine  "A"  so  that  the  bus  bar  voltage 
remains  constant.     Then  turn  on  enough  lamps  to  load  each  of 
the  machines  to  its  full  rated  capacity. 

6.  With  both  machines  fully  loaded,  observe  and  record  the 
bus  bar  voltage,  current  delivered  by  each  machine  and  total 
current  taken  from  the  bus  bars. 

7.  Leaving  the  field  rheostats  untouched,  repeat  the  observa- 
tions of  item  6  for  %,  %  and  %  of  the  total  load  current  and 
for  zero  current  from  the  bus  bars  in  turn. 

8.  Turn  on  the  lamps  and  vary  the  field  rheostats  and  lamps 
until  each  machine  delivers  one-half  of  its  rated  load.     Adjust 
the  field  rheostat  of  the  two  machines  until  the  machine  "B"  is 
delivering  all  the  current  and  machine  "A"  is  unloaded,  main- 
taining constant  voltage  at  the  bus  bars  throughout  this  adjust- 
ment.   Now  disconnect  machine  "  A  "  from  the  bus  bars. 

Written  Report. — 1.  What  would  be  the  result  if  the  negative 
terminal  of  machine  "B"  was  connected  by  mistake  to  the  posi- 
tive terminal  of  the  bus  bars  in  item  4,  Order  of  Work  ? 

2.  From  the  observations  in  items  6  and  7,  in  which  of  the  two 
machines  is  the  armature  voltage  reduced  the  more  as  the  total 
load  is  decreased? 

3.  In  item  8,  why  must  all  the  load  be  shifted  to  machine  "  B  " 
before  machine  "A"  is  disconnected  from  the  bus  bars? 

4.  What  would  have  been  the  result  if  in  item  4,  Order  of 
Work,  machine  "B"  had  been  connected  to  the  bus  bars  when 


DIRECT  CURRENT  65 

its  induced  voltage  was  much  above  that  of  the  bus  bars?    If  it 
was  lower  ? 

5.  Explain  any  inequalities  observed  in  the  sharing  of  the  total 
bus  bar  load  by  the  two  machines  as  the  total  output  was  reduced 
in  item  7.  If  any  inequalities  existed,  to  what  were  they  due? 
Explain. 

EXPERIMENT  20. 
Compound  Generators  in  Parallel. 

See  Article  122  in  the  text  book,  also  the  Theory  under  Experi- 
ment 19  in  the  Manual. 

The  object  of  this  experiment  is  to  observe  the  factors  involved 
in  the  parallel  operation  of  compound  generators,  first,  in  regard 
to  the  conditions  necessary  before  throwing  one  generator  in 
parallel  with  another  machine,  and  second,  in  regard  to  the  items 
which  influence  the  equal  or  proportionate  sharing  of  the  total 
output  of  a  power  station  by  the  various  generators  connected  in 
parallel  for  supplying  this  total  station  output. 

Theory. — As  stated  in  experiment  19,  the  general  practice  in 
electric  stations  is  to  supply  power  from  bus  bars  to  which  are 
connected  a  number  of  generators  in  parallel  with  each  other, 
each  delivering  its  share  of  the  toal  load  supplied  from  the  com- 
mon bus  bars.  In  this  way  it  is  possible  to  operate  the  machines 
at  a  relatively  high  efficiency  even  when  the  station  output  is  low, 
by  disconnecting  certain  machines  and  thus  keeping  the  remain- 
ing machines  in  operation  at  or  near  full  load.  In  practice  the 
generators  thus  used  are  compound  wound  machines. 

As  in  the  parallel  operation  of  shunt  generators,  where  the  cur- 
rent delivered  by  a  given  machine  depends  on  the  difference  be- 
tween its  induced  voltage  and  the  bus  bar  voltage,  so  in  the 
parallel  operation  of  compound  generators  the  difference  between 
the  induced  voltage  in  a  given  machine  and  the  bus  bar  voltage 
determines  the  amount  of  current  supplied  by  the  machine.  The 
induced  voltage  in  a  shunt  generator  depends  primarily  on  the 
field  strength  and  on  the  speed  of  the  machine.  In  the  compound 
generator,  however,  the  induced  voltage  depends  not  only  on  the 
field  strength  produced  by  the  shunt  winding  and  on  the  speed, 
6 


66  LABORATORY  MANUAL 

but  also  on  the  additional  field  produced  by  the  series  winding. 
Hence,  the  equal  or  proportionate  sharing  of  the  load  with  com- 
pound generators  depends  on  an  additional  factor,  namely,  the 
strength  of  the  field  magnetism  produced  by  the  series  field  cur- 
rent. 

If  two  compound  generators  are  connected  in  parallel  (in  gen- 
eral as  shown  in  Fig.  19,  except  that  a  series  winding  is  inserted 
between  the  upper  armature  terminal  and  the  machine  ammeter 
in  each  case),  each  machine  may  be  made  to  deliver  its  share  of 
a  given  total  bus  bar  load  by  adjusting  its  shunt  field  rheostat. 
If  the  machines  are  over-compounded,  however,  and  one  machine 
speeds  up  due  to  an  increase  in  the  speed  of  its  driving  engine, 
this  means  an  increase  in  the  induced  voltage  (due  to  the  in- 
crease in  speed).  The  output  of  this  machine  is  then  increased 
(meaning,  of  course,  that  the  output  of  the  second  machine  falls 
off  by  a  corresponding  amount)  and  the  action  of  the  series  wind- 
ing is  to  increase  still  further  the  induced  voltage  in  the  first 
machine.  This  condition  being  cumulative,  results  in  an  ex- 
cessive overload  for  the  first  machine  and  obviously  in  an  Unbal- 
anced condition  of  operation.  (This  effect  of  instability  is  more 
noticeable  with  over-compounded  than  with  flat  or  under-com- 
pounded generators.) 

To  prevent  this  unstable  condition  an  equalizer  connection  is 
made  between  the  two  machines,  at  the  junction  of  the  series 
winding  and  the  armature  terminal  in  each  case  (for  the  short 
shunt  connection),  and  this  places  the  two  series  windings  in 
parallel  with  each  other  as  regards  the  output  current  from  the 
machines,  thus  insuring  that  the  series  field  current  will  be  in- 
versely proportional  to  the  resistance  of  the  windings  at  all  times, 
irrespective  of  any  tendency  for  the  armature  currents  to  be 
unequal. 

If  the  compounding  of  the  machines  is  different,  thus  causing 
an  unequal  sharing  of  a  given  total  bus  bar  load,  the  resistance  of 
the  series  field  circuit  must  be  changed  by  connecting  an  auxili- 
ary resistance  in  series  with  the  series  winding,  thus  reducing  the 
part  of  the  total  current  which  flows  through  the  series  winding 
of  the  first  machine  and  allowing  more  current  to  flow  through 
the  series  winding  of  the  second  machine. 

Adjustments  of  the  compounding  by  means  of  a  shunt  around 
the  series  field,  as  explained  in  Experiment  11,  will  not  serve  the 


DIRECT  CURRENT  67 

purpose  in  this  case  on  account  of  the  parallel  condition  of  the 
two  series  windings.  (Note:  The  student  should  make  a  dia- 
gram, similar  to  Fig.  19,  on  the  data  sheet  with  the  series  fields 
arranged  for  the  short  shunt  connection,  and  verify,  theoretically, 
the  instability  of  operation  without  an  equalizer  connection.  Al- 
so, with  an  equalizer  connection  on  the  diagram,  the  statements 
in  the  two  preceding  paragraphs  should  be  verified  before  the 
experiment  is  undertaken. ) 

Current  Supply. — From  the  Compound  Generators  assigned. 

Apparatus  Required. — (1)  Two  compound  generators  (prefer- 
ably over-compounded)  ;  (2)  lamp  banks  to  be  used  as  a  common 
load  supplied  from  the  bus  bars;  (3)  field  rheostats  for  each  ma- 
chine; (4)  a  double-pole  single-throw  switch  for  each  machine 
and  for  the  total  load  (3  in  all)  ;  (5)  ammeters,  one  for  each  ma- 
chine, one  for  the  total  output  current  from  the  bus  bars,  and  one 
to  be  connected  in  the  equalizer  circuit  between  the  two  machines 
(the  latter  instrument  preferably  a  double-throw  ammeter  to  in- 
dicate the  current  no  matter  in  which  direction  it  flows)  ;  (6) 
two  voltmeters,  one  for  the  bus  bars  and  one  for  the  on-coming 
machine;  (7)  equalizer  connection  (a  wire  to  be  connected  be- 
tween the  two  machines  at  the  junction  of  the  series  field  and  the 
armature  in  each  case,  where  the  short  shunt  connection  is  used ) . 

Order  of  Work. — 1.  Arrange  the  connections  of  the  two  assigned 
compound  generators  similar  to  Fig.  19,  except  that  the  series 
field  is  to  be  connected  between  the  upper  armature  terminal  and 
the  ammeter  in  each  case,  and  the  equalizer  is  to  be  connected 
between  the  upper  armature  terminals  of  the  two  machines 
through  an  ammeter. 

2.  With  all  switches  open,  start  the  two  generators  and  adjust 
the  voltage  of  each  to  its  normal  value. 

3.  Connect  one  of  the  generators  ("A")  to  the  bus  bars  (two 
lengths  of  wire)  and  turn  on  enough  lamps  to  load  the  machine 
to  its  full  capacity. 

4.  Adjust  the  voltage  of  the  other  generator  ("B")   to  the 
same  value  as  the  bus  bar  voltage  (or  a  trifle  lower)  and  connect 
it  to  the  bus  bars,  being  sure  that  the  positive  terminal  of  the 
machine  is  connected  to  the  positive  terminal  of  the  bus  bars. 
(Remember  that  as  soon  as  machine  "B"  is  thrown  on  to  the  bus 
bars,  a  current  will  flow  through  its  series  field  from  machine 


68  LABORATORY  MANUAL 

11  A"  which  will  tend  to  increase  the  induced  voltage  in  the  arma- 
ture of  "B".  This  is  the  reason  for  adjusting  the  voltage  of 
"B"  a  trifle  lower  than  the  bus  bar  voltage  before  connecting  the 
two.  Watch  the  ammeter  of  machine  "B"  carefully  until  the  ad- 
justments of  load  have  been  made  according  to  the  following 
item.) 

5.  Vary  the  shunt  field  rheostat  of  machine  "B"  until  the  cur- 
rent of  the  two  machines  is  the  same  in  value,  at  the  same  time 
adjusting  the  shunt  field  rheostat  of  machine  "A"  so  that  the 
bus  bar  voltage  remains  constant.    Then  turn  on  enough  lamps  to 
load  each  machine  to  its  full  rated  capacity. 

6.  With  both  machines  fully  loaded,  observe  and  record  the 
bus  bar  voltage,  current  delivered  by  each  machine,  total  current 
taken  from  the  bus  bars,  and  the  equalizer  current  (noting  in 
which  direction  the  equalizer  current  flows,  that  is,  whether  from 
machine  "A"  to  "B",  or  from  "B"  to  "A".) 

7.  With  the  field  rheostats  untouched,  repeat  the  observations 
called  for  in  item  6,  for  %,  %,  and  a/4  of  the  total  load,  and  for 
zero  current  from  the  bus  bars  in  turn. 

8.  Turn  on  the  lamps  and  vary  the  field  rheostats  and  lamps 
until  each  machine  delivers  one-half  of  its  rated  load.     Adjust 
the  field  rheostat  of  the  two  machines  until  machine  "B"  is  de- 
livering all  the  current  and  machine  "A"  is  unloaded,  maintain- 
ing constant  voltage  at  the  bus  bars  throughout  the  adjustment. 
Now  disconnect  machine  "A"  from  the  bus  bars. 

9.  Same  as  items  6  and  7,  with  a  resistance  inserted  in  one  of 
the  series  field  circuits,  so  that  when  the  load  from  the  bus  bars 
equals  the  total  rated  capacity  of  the  two  machines  combined, 
machine  "A"  is  delivering  l1/^  of  its  rated  capacity  and  machine 
"B"  %  of  its  rated  capacity. 

Written  Report. — 1.  Explain  why  the  operation  of  compound 
generators  in  parallel  is  unstable  without  an  equalizer  connec- 
tion. 

2.  Why  is  this  unstable  condition  less  noticeable  for  flat  and 
under-compounded  than  for  over-compounded  machines  ? 

3.  What  current  was  indicated  by  the  equalizer  circuit  am- 
meter in  items  5,  6  and  7,  Order  of  Work?    If  any,  to  what  was  it 
due? 


DIRECT  CURRENT  69 

4.  Explain  any  inequalities  observed  in  the  sharing  of  the 
total  bus  bar  load  by  the  two  machines,  as  the  total  output  was 
reduced  in  items  6,  7  and  9.    If  inequalities  existed,  to  what  were 
they  due?    Explain. 

5.  Explain  the  use  of  resistance  in  series  with  one  of  the  series 
field  windings  in  item  9.     How  did  it  affect  the  proportionate 
sharing  of  the  total  output  by  the  two  machines  as  the  total  out- 
put was  reduced  ? 

6.  Why  cannot  a  shunt  around  the  series  field  be  used  for  the 
purpose  of  adjusting  the  sharing  of  the  total  output  in  item  9  in- 
stead of  a  resistance  in  series  with  the  series  field  winding  ?  What 
is  the  function  of  the  resistance  as  here  used?    Explain. 


Experiments  21  to  30,  inclusive,  constitute  the 
Alternating    Current    portion    of    the    Manual. 


ALTERNATING  CURRENT 

EXPERIMENT  21. 
Resistance  and  Reactance  in  Series. 

See  Articles  261,  269,  270,  271,  272,  277,  278  and  279  in  the 
text  book. 

The  object  of  this  experiment  is  to  make  a  study  of  the  volt- 
age, current  and  power  relations  in  a  simple  alternating  current 
circuit  containing  both  resistance  and  reactance  in  series. 

Theory. — Ohm's  law  states  that  in  a  circuit,  or  portion  of  a 
complete  circuit,  where  all  the  voltage  goes  to  overcome  resistance 
only,  the  current  (7)  equals  E/R,  where  E  is  the  electromotive 
force  across  the  terminals  of  the  circuit  and  R  is  the  resistance  of 
the  circuit  in  ohms.  This  law  holds  true  for  both  direct  and  al- 
ternating current  circuits  as  regards  that  portion  of  E  which 
overcomes  resistance  (R)  only. 

In  a  direct  current  motor,  the  voltage  (E±)  across  the  termi- 
nals of  the  armature  is  partly  used  to  overcome  resistance  and 
partly  to  overcome  the  counter  electromotive  force  induced  in  the 
armature  by  the  rotation  of  the  armature  wires  in  the  magnetic 
field,  that  is,  E^  (the  impressed  electromotive  force)  =RI-\-E2 
(the  counter  electromotive  force  of  the  armature).  Note  that  the 
RI  drop  and  E2  are  added  numerically. 

In  an  alternating  current  circuit  consisting  of  a  coil  of  wire, 
the  voltage  (Er)  across  the  terminals  of  the  coil  is  partly  used  to 
overcome  resistance  and  partly  to  overcome  the  counter  electro- 
motive force  induced  in  the  coil  by  the  rapid  reversals  of  the  mag- 
netic field  in  the  coil  due  to  the  alternating  current,  that  is,  E^ 
(the  impressed  electromotive  force)  =RI  added  vectorially  to  E2 
(the  counter  electromotive  force  in  the  coil).  Note  that  the  RI 
drop  and  E2  are  added  vectorially  (not  numerically)  because 
they  are  90°  apart  in  phase.  E2,  the  counter  electromotive  force, 
is  usually  expressed  as  XI,  that  is,  the  reactance  (X)  of  the  cir- 

71 


72  LABOR  A  TORY  MANUAL 

cult  times  the  current  (7),  X  being  the  opposition  due  to  the  in- 
ductance of  the  coil,  but  for  convenience  expressed  in  ohms  like 
the  resistance  R.  (Read  carefully  Articles  165  and  171  in  the 
text  book. ) 

Since  the  reactance  of  a  circuit  depends  on  the  frequency  of 
the  alternating  electromotive  force  across  its  terminals,  the  oppo- 
sition due  to  reactance  is  high  for  high  frequencies  and  low  for 
low  frequencies  in  a  given  circuit.  Obviously  the  frequency  has 
nothing  whatever  to  do  with  the  RI  drop  in  a  circuit.  Hence, 
if  an  alternating  current  of  10  amperes  flows  in  a  circuit  contain- 
ing both  R  and  X  when  110  volts  at  60  cycles  are  applied  at  its 
terminals,  if  the  frequency  is  reduced  to  30  cycles,  and  the  cur- 
rent maintained  at  10  amperes  by  reducing  the  electromotive 
force,  the  RI  component  of  E  will,  of  course,  remain  the  same, 
while  the  XI  component  will  be  i/2  as  great  as  before  because  X 
is  %  its  former  value.  A  coil  containing  both  resistance  and  re- 
actance produces  the  same  result  as  a  separate  resistance  and  a 
separate  reactance  connected  in  series. 

Since  the  voltage  required  to  overcome  R  and  X  is  made  up  of 
two  components  90°  apart  in  phase,  the  combined  effect  of  the 
two  (R  and  X)  may  be  expressed  as  Z,  which  equals  the  square 
root  of  (R2-{-X2).  Z  (expressed  in  ohms)  is  usually  termed  the 
impedance  of  the  circuit,  and  it  represents  the  total  opposition 
to  the  flow  of  current  in  an  alternating  current  circuit  due  both 
to  resistance  and  to  counter  electromotive  force  (or  reactance). 

In  a  direct  current  circuit  containing  several  resistances  in 
series  when  a  current  (/)  flows,  the  volts  drop  across  the  vari- 
ous resistances  added  together  numerically  determine  the  total 
voltage  of  the  circuit.  Similarly,  in  an  alternating  current  cir- 
cuit, containing  both  resistance  (R)  and  reactance  (X)  in  series, 
the  volts  drop  across  the  various  resistances  and  reactances  are 
added  vectoriaUy  to  determine  the  total  voltage  of  the  circuit. 

In  the  direct  current  circuit  the  power  in  the  circuit  equals 
the  electromotive  force  (E)  times  the  current  (7)  because  both 
E  and  7  may  be  thought  of  as  in  the  same  direction  at  all  times. 
In  the  alternating  current  circuit,  the  power  in  the  circuit  also 
equals  El  in  those  cases  where  E  and  7  are  in  the  same  direction 
at  all  times  (that  is,  in  phase  with  each  other),  as  in  a  circuit 
containing  resistance  only.  Where  E  and  7  are  not  in  the  same 
direction  at  all  times,  (that  is,  out  of  phase)  as  in  circuits  con- 


ALTERNATING  CURRENT 


73 


taining  reactance  as  well  as  resistance,  account  must  be  taken  of 
the  fact  that  E  and  7  are  out  of  phase  by  an  angle  ''a",  and  the 
product  El  must  be  multiplied  by  a  factor  called  the  power  fac- 
tor (cos  a)  of  the  circuit.  A  wattmeter,  however,  indicates  the 
true  power  (El  cos  a)  at  all  times,  even  in  those  cases  where  the 
voltmeter  reading  (E)  and  the  ammeter  reading  (/)  taken  to- 
gether do  not  indicate  the  true  power. 


Supply  Mains 

(110  Volts  60  Cycles  A.  C.) 

A 

—  r  i        r  I  — 

Voltmeter 

C=? 

"R"  (Lamps  in  Parallel) 
Adjustable  Resistance 

Fig.  19. — Study  of  the  voltage  and  current  relations  in  series  cir- 
cuits, made  up,  in  this  case,  of  a  reactance  coil  and  a  lamp  bank  in 
series.  The  adjustable  resistance  shown  to  the  left  is  an  auxiliary  to 
the  apparatus  under  test. 


In  a  circuit  like  that  of  Fig.  19,  where  a  coil,  with  both  resist- 
ance and  reactance  is  connected  in  series  with  a  resistance  (lamp 
bank),  the  current  (7)  is  obviously  the  same  throughout  the  cir- 
cuit and  the  voltage  E3  across  the  entire  circuit  is  equal  to  the  RI 
drop  (EJ  across  the  lamps  added  vectorially  to  the  voltage  (E2) 
across  the  coil.  The  relation  of  these  3  voltages,  each  of  which 
may  be  measured  separately  by  a  voltmeter,  is  shown  in  Fig.  20. 
This  diagram  also  shows  graphically  how  the  reactance  (X)  of 
the  coil  may  be  determined  from  the  voltage  readings. 

Current  Supply. — 110  volts  50  and  60  cycle  Alternating  Cur- 
rent and  110  volts  Direct  Current. 

Apparatus  Required. —  (1)  Reactance  coil;  (2)  circuit  contain- 
ing resistance  only,  for  example,  a  lamp  bank;  (3)  voltmeter 


74  LABORATORY  MANUAL 

with  rather  large  range;  (4)  voltmeter  with  low  range  (for  meas- 
uring the  RI  drop  when  Direct  Current  is  used)  ;  (5)  ammeter; 
and  (6)  wattmeter. 

Order  of  Work. — 1.  Connect  the  coil  and  the  resistance  (R)  in 
series  as  shown  in  Fig.  19  (the  auxiliary  resistance  is  put  in  to 
permit  of  current  adjustments).  Adjust  the  auxiliary  resist- 
ance until  a  fair  value  of  current  flows  from  the  110-volt,  60- 
cycle  Alternating  Current  mains.  Keeping  this  current  constant. 


(Resistance  R)  RI  in  Coil 


a  =  Phase  Difference  Between  ES  and  I 

Fig.  20. — Vector  relations  of  the  voltages  and  current  in  series  cir- 
cuits, as  in  Fig.  19. 

observe  and  record  the  volts  across  the  coil,  across  the  resistance 
(R),  the  total  volts,  not  including  the  auxiliary  resistance,  cur- 
rent, frequency,  and  total  watts,  not  including  the  auxiliary  re- 
sistance. Use  Form  12. 

2.  Connect  the  same  coil  and  resistance  (R)  to  the  110-volt 
Direct  Current  mains,  and  reduce  the  voltage  across  the  coil 
and  resistance  (R)  by  the  auxiliary  resistance  until  the  current 
is  the  same  value  as  in  item  1.     Observe  and  record  the  same 
readings  called  for  in  item  1. 

3.  Same  as  item  1,  except  that  R  is  to  have'l  2/3,  1 1/3,  2/3 
and  then  1/3  the  original  value  in  turn,  adjust  to  the  same  cur- 
rent as  in  item  1  in  each  case,  and  repeat  the  observations  called 
for  in  item  1. 

4.  Connect  the  coil  and  the  resistance  (R)  as  in  item  1  to  50 
(or  less)  cycle  Alternating  Current  mains  and  adjust  the  current 


ALTERNATING  CURRENT 


75 


until  it  is  the  same  value  as  in  items  1  and  2. 
servations  called  for  in  item  1. 


Take  the  same  ob- 


Written  Report. — 1.  From  the  observations  in  item  1,  Order  of 
Work,  draw  a  diagram  similar  to  Fig.  20,  and  from  this  deter- 
mine graphically  the  reactance  volts  (XI)  drop  of  the  coil  and 
the  phase  difference  between  Es  and  I  in  degrees;  also  calculate 
the  reactance  (X)  in  ohms  and  the  power  factor  of  the  entire  cir- 
cuit not  including  the  auxiliary  resistance.  The  power  factor= 
watts  (indicated  by  wattmeter)  /EJ,  that  is,  true  watts  divided 
by  apparent  watts. 


No. 

Supply 
Current 

Volts 

Amperes 

Frequency 

Watts 
(Total) 

"R" 

Coil 

Total 

1 

(A.  C.) 

2 

(D.  C.) 

3 

(A.  C.) 

Form  12. 

2.  Is  EZI  in  item  2,  Order  of  Work,  the  same  in  value  as  the 
wattmeter  reading  in  item  1,  Order  of  Work,  for  the  same  value 
of  current  (/)  in  each  case?    If  so,  why?    If  not,  why? 

3.  Draw  a  vector  diagram,  similar  to  Fig.  20,  for  the  voltages 
observed  in  items  3  and  4,  Order  of  Work,  and  repeat  the  require- 
ments under  1,  Written  Report. 

4.  How  do  the  reactance  volts  drop  in  items  1  and  4,  Order  of 
Work,  compare? 

(Note:  The  impedance  of  an  alternating  current  circuit  often 
involves  reactance  due  to  capacity  as  well  as  to  inductance,  but 
for  simplicity  this  experiment  has  been  limited  to  the  effect  due 
to  resistance  and  inductive  reactance  only.) 


76 


LABORATORY  MANUAL 


EXPERIMENT  22. 
Resistance  and  Reactance  in  Parallel. 

See  Article  273  in  the  text  book,  also  the  Theory  under  Ex- 
periment 21  in  the  Manual. 

The  object  of  this  experiment  is  to  make  a  study  of  the  volt- 
age, current  and  power  relations  in  a  simple  alternating  current 
circuit  containing  both  resistance  and  reactance  in  parallel. 


Supply  Mains 

(110  Volts  60  Cycles  A    C.) 

—  r        nb— 

Fig.  21. — Study  of  the  current  and  voltage  relations  in  parallel  cir- 
cuits, made  up,  in  this  case,  of  a  reactance  coil  and  a  lamp  bank.  Note 
that  the  two  ammeters  to  the  right  measure  the  current  in  the  coil  and 
in  the  lamp  bank,  respectively. 

Theory. — In  direct  current  circuits  where  a  number  of  resist- 
ances are  connected  in  series,  the  current  is  the  same  throughout 
the  circuit,  while  the  volts  drop  across  the  separate  resistances 
are  added  numerically  to  determine  the  total  voltage  of  the  cir- 
cuit. In  the  alternating  current  circuit,  where  resistances  and 
reactances  are  connected  in  series  the  current  is  also  obviously 
the  same  throughout  the  circuit,  while  the  volts  drop  across  the 
various  parts  of  the  circuit  are  added  vectorially  to  determine 


ALTERNATING  CURRENT  77 

the  total  voltage.     These  have  been  investigated  in  experiment 
21. 

In  direct  current  circuits  where  a  number  of  resistances  are 
connected  in  parallel,  the  voltage  is  obviously  the  same  across 
each  of  the  resistances,  while  the  total  current  is  equal  to  the 
numerical  sum  of  the  individual  currents  in  the  various  resist- 
ances. In  the  alternating  current  circuit,  where  resistances  and 
reactances  are  connected  in  parallel,  the  voltage  is  the  same 
across  the  terminals  of  each  portion  of  the  circuit,  while  the  total 
current  is  equal  to  the  vector  sum  of  the  individual  currents 
through  the  separate  parts  of  the  circuit. 


a  =  Phase  Difference  Between  E  and   Is 

Fig.  22. — Vector  relations  of  the  currents  and  voltage  in  parallel  cir- 
cuits, as  in  Pig.  21. 

In  a  circuit  like  that  of  Fig.  21,  where  a  coil  with  both  resist- 
ance and  reactance  is  connected  in  parallel  with  a  resistance 
(lamp  bank),  the  voltage  (E)  is  the  same  for  both  parts  of  the 
circuit,  and  the  total  current  (73)  is  equal  to  the  current  72  in  the 
coil  added  veetorially  to  the  current  7X  in  the  lamp  bank.  The 
diagram  shown  in  Fig.  22  indicates  the  relations  of  these  three  cur- 
rents, each  of  which  may  be  measured  separately  by  an  ammeter. 
From  this  diagram  the  phase  difference  between  7X,  72  and  73  may 
be  determined  graphically. 

Current  Supply. — 110  volts  50  and  60  cycle  Alternating  Cur- 
rent and  110  volts  Direct  Current. 


78  LABORATORY  MANUAL 

Apparatus  Required. —  (1)  Reactance  coil;  (2)  circuit  contain- 
ing resistance  only  (lamp  bank)  ;  (3)  ammeters,  one  for  each  cir- 
cuit and  one  for  the  total  current;  (4)  voltmeter;  and  (5)  watt- 
meter. 

Order  of  Work. — 1.  Connect  the  coil  and  the  lamp  bank  in  par- 
allel to  the  60-cycle  mains,  as  shown  in  Fig.  21,  with  due  care 
that  the  current  through  the  coil  is  not  excessive.  Observe  and 
record  the  current  in  the  coil,  in  the  lamp  bank  and  the  total 
current,  volts,  frequency  and  total  watts. 

2.  Connect  the  coil  and  lamp  bank  to  the  110-volt  Direct  Cur- 
rent mains,  adjusting  the  voltage  if  necessary  to  the  same  value 
as  used  in  item  1.     Observe  and  record  the  same  readings  called 
for  in  item  1.     (Note:    If  the  current  through  the  coil  is  exces- 
sive with  the  Direct  Current,  insert  a  protective  resistance  in 
series  with  it  to  bring  down  the  current  to  a  normal  value,  and 
repeat  item  1  with  this  extra  resistance  in  circuit  so  as  to  have 
the  voltage  conditions  the  same  in  both  items  1  and  2.) 

3.  Same  as  item  1,  except  that  R  is  to  have  1 1/3  and  2/3  the 
original  value  in  turn,  use  the  same  voltage  as  in  item  1  in  each 
case,  and  repeat  the  observations  called  for  in  item  1. 

4.  Same  as  item  1,  except  that  a  reactance  coil  with  a  different 
power  factor  from  the  original  coil  is  to  be  substituted  for  the 
lamp  bank.    Use  the  same  voltage  as  in  item  1,  and  repeat  the  ob- 
servations called  for  in  item  1. 

5.  Connect  the  coil  and  the  lamp  bank  to  the  50-cycle  mains, 
and  adjust  the  voltage  to  the  same  value  as  in  items  1  and  2. 
Take  the  same  observations  called  for  in  item  1. 

Written  Report. — 1.  From  the  observations  in  item  1,  Order  of 
Work,  draw  a  diagram  similar  to  Fig.  22,  and  from  this  deter- 
mine graphically  the  phase  difference  between  E  and  73  in  de- 
grees and  calculate  the  power  factor  of  the  entire  circuit  (true 
watts  divided  by  apparent  watts). 

2.  Does  EI^    (Direct  Current)    equal  EI^    (Alternating  Cur- 
rent) in  items  1  and  2,  Order  of  Work?    If  so,  why?     If  not, 
why? 

3.  Same  as  item  1,  Written  Report,  for  items  3  and  4,  Order 
of  Work. 


ALTERNATING  CURRENT  79 

4.  Draw  a  vector  diagram  similar  to  Fig.  22  for  the  current 
values  observed  in  item  5,  Order  of  Work,  and  repeat  the  require- 
ments called  for  under  item  1,  Written  Report. 

5.  How  do  the  current  values  through  the  coil  compare  in  items 
1,  3  and  5,  Order  of  Work? 

6.  Does  EI.A  in  item  2,  Order  of  Work,  equal  the  wattmeter 
reading  in  item  1,  Order  of  Work?    If  so,  why  ?    If  not,  why? 

(Note:  The  impedance  of  an  alternating  current  circuit  often 
involves  reactance  due  to  capacity  as  well  as  to  inductance,  but 
for  simplicity  this  experiment  has  been  limited  to  the  effect  due 
to  resistance  and  inductive  reactance  only.) 


EXPERIMENT  23. 
Study  of  Three-Phase  Circuits. 

See  Article  282  in  the  text  book. 

The  object  of  this  experiment  is  to  afford  an  opportunity  for 
observing  the  voltage  and  current  relations  in  three-phase  cir- 
cuits. (Note:  Two-phase  circuits  are  somewhat  simpler  and  the 
relations  between  phases  perhaps  more  readily  understood,  hence, 
where  the  laboratory  apparatus  is  two-phase,  the  following  ex- 
periment may  be  carried  out  with  the  two-phase  instead  of  the 
three-phase  apparatus  as  here  suggested.) 

Theory. — As  three-phase  alternating  current  transmission  of 
electric  power  is  very  generally  used  over  long  distances,  and 
further,  since  many  three-phase  induction  motors  are  in  service, 
the  general  relations  of  voltage  and  current  in  such  circuits  are 
of  special  interest. 

A  single  winding  on  the  armature  of  an  alternating  current 
generator  with  two  collector  rings  for  delivering  the  current,  is 
called  a  single-phase  machine.  Two  electrically  separate  wind- 
ings may  be  used  instead  of  one,  each  winding  terminating  in  a 
set  of  two  collector  rings  and  thus  making  a  two-phase  machine. 
The  two  windings  in  such  a  case  are  wound  with  a  definite  angu- 
lar displacement  between  corresponding  wires,  this  displacement 
being  such  that  the  electromotive  forces  in  the  two  coils  differ 
by  90°  in  phase.  Similarly,  three  electrically  separate  coils  may 
be  wound  on  the  armature  for  making  a  three-phase  generator, 


80 


LABORATORY  MANUAL 


the  displacement  of  the  wires  of  the  three  windings  being  such 
that  the  electromotive  forces  differ  in  phase  by  120°  from  each 
other. 

The  principal  advantages  of  the  three-phase  as  compared  with 
the  single-phase  current  are  first,  the  economy  in  the  amount  of 
copper  wire  required  for  the  transmission  of  a  given  amount  of 
power  by  the  three-phase  scheme,  and  second,  the  improved  con- 


Fig.  23. — Study  of  the  voltage  and  current  relations  in  a  three-phase 
"Y"  (or  Star)  connected  receiving  circuit. 

ditions  afforded  by  three-phase  currents  in  the  operation  of  in- 
duction motors,  and  other  apparatus. 

The  six  terminals  of  a  three-phase  armature  may  either  be  con- 
nected for  the  so-called  Y  (or  Star)  or  the  Delta  connection,  and 
similarly  the  three  component  parts  of  the  receiving  circuit  for 
the  three-phase  line  may  either  be  connected  for  Y  or  for  Delta 
operation. 

From  Fig.  406,  page  512  of  the  text  book,  showing  a  T  con- 
nected system,  it  can  readily  be  seen  that  the  electromotive  force 
between  any  two  of  the  outside  line  wires,  X,  Y  and  Z  (the  wire 


ALTERNATING  CURRENT 


81 


connected  to  the  middle  point  is  generally  omitted  in  practice) 
is  the  vector  sum  of  the  electromotive  forces  induced  in  the  two 
coils  connected  in  series  between  these  two  line  wires.  Further, 
as  the  electromotive  forces  in  series  are  60°  apart  in  phase,  the 
vector  sum  of  the  two  is  equal  to  the  V3  (=1.73)  times  that  in 
one,  since  the  electromotive  forces  are  the  same  in  each  of  the 
three  armature  coils,  and  as  explained  in  the  text  book.  The  cur- 


Fig.  24. — Study  of  the  voltage  and  current  relations  in  a  three-phase 
"Delta"  connected  receiving  circuit. 

rent  in  any  one  of  the  outside  line  wires,  however,  is  supplied  by 
one  armature  coil  only,  as  will  be  seen  by  an  inspection  of  the 
illustration.  Hence,  the  current  in  the  line  wire  and  in  the  arma- 
ture coil  to  which  it  is  connected,  is  the  same  in  value. 

From  Fig.  408,  page  513  of  the  text  book,  showing  a  Delta 
connected  system,  it  will  be  seen  that  the  electromotive  force  be- 
tween any  two  of  the  line  wires,  X,  Y  and  Z,  is  equal  to  the  elec- 
tromotive force  induced  in  the  armature  coil,  shown  in  the  upper 
part  of  the  figure,  connected  between  the  terminals  of  the  two  line 
wires.  Further,  since  any  one  of  the  line  wires  receives  current 

7 


82 


LABORATORY  MANUAL 


from  the  two  coils  terminating  at  its  connection  to  the  armature 
circuit,  the  current  in  any  one  line  wire  is  the  vector  sum  of  the 
currents  in  the  two  coils  or  the  \/3  times  the  current  in  one  coil 
(since  the  current  has  the  same  value  in  each  of  the  three  arma- 
ture coils). 

Hence,  if  E  and  7  be  the  voltage  and  current  respectively  in  a 
single  armature  coil  of  a  Y  connected  generator,  the  voltage  be- 
tween any  two  line  wires  is  V3#,  while  the  current  per  line  is 
simply  I.  "With  a  Delta  connection,  this  same  machine  would 
produce  a  voltage  E  between  any  two  line  wires  and  a  current 
per  line  equal  to  \/3L  Thus  a  given  number  of  turns  on  the  ar- 
mature of  a  three-phase  generator  results  in  a  higher  line  volt- 


No 

Connection  Used 

Amperes 

Volts 

Lamp  Bank 
"1" 

Lamp  Bank 
»2" 

Lamp  Bank 
"3" 

Line 

Lamp  Bank 
"1" 

Lamp  Bank 

"2" 

Lamp  Bank 
"3" 

Line 

1 

"Y" 

2 

,,Y, 

3 

"Delta* 

Form  13. 

age  if  Y  than  if  Delta  connected.  Note  that  the  total  power  is 
the  same  for  the  given  machine  no  matter  which  way  it  is  con- 
nected, since  it  is  expressed  by  ^3EI  in  each  case  (assuming 
unity  power  factor). 

Current  Supply. — Three-phase  Alternating  Current. 

Apparatus  Required. —  (1)  Three  similar  receiving  circuits 
(lamp  banks)  ;  (2)  four  ammeters,  one  for  each  receiving  cir- 
cuit and  one  for  a  single  line;  (3)  voltmeter. 

Order  of  Work. — 1.  Arrange  the  three  lamp  banks  as  a  Y  con- 
nected receiving  circuit  to  be  supplied  by  a  three-phase  generator 
as  shown  in  Fig.  23.  Adjust  the  current  so  as  to  be  equal  in  each 
of  the  three  lamp  banks,  and  observe  and  record  the  current  in 
each  of  the  three  lamp  banks  and  in  one  of  the  lines,  also  the 
voltage  between  any  two  line  w^ires  and  across  each  of  the  three 
lamp  banks.  Use  Form  13, 


ALTERNATING  CURRENT  83 

2.  Same,  for  twice  the  current  value  in  each  of  the  lamp  banks. 

3.  Arrange  the  three  lamp  banks  as  a  Delta  connected  receiv- 
ing circuit,  as  shown  in  Fig.  24,  and  repeat  the  observations 
called  for  in  items  1  and  2. 

4.  Ascertain  whether  the  generator  armature  windings  are 
connected  Y  or  Delta. 

5.  Observe  the  number  of  terminals  in  one  of  the  three-phase 
induction  motors  in  the  laboratory. 

Written  Report. — 1.  From  the  observations  in  items  1  and  2, 
Order  of  Work,  calculate  the  numerical  relation  between  line 
and  individual  receiving  circuit  voltage  and  current.  Record 
these  calculations  in  the  Form  on  the  data  sheet. 

2.  Same,  for  item  3,  Order  of  Work. 

3.  When  the  armature  terminals  of  a  three-phase  generator 
are  arranged  for  the  Y  connection,  where  are  the  connections 
made  in  the  machine  ?    Explain. 

4.  Same,  for  Delta  connected  armature  winding.    Explain. 

5.  In  Fig.  406,  page  512  of  the  text  book,  a  fourth  wire  is 
shown  tapped  to  the  common  intersection  of  each  of  the  Y  con- 
nected armature  windings.     Why  can  this  fourth  wire  usually 
be  omitted  in  practical  cases  ? 

6.  In  the  three-phase  Delta  connected  armature  winding,  why 
is  it  that  there  is  no  circulation  of  current  about  the  three  wind- 
ings which  are  in  reality  connected  in  series  as  a  closed  loop  ? 

7.  Why  is  the  current  per  line  in  a  Delta  connected  circuit 
equal  to  the  \/3  times  the  current  in  one  armature  winding? 


EXPERIMENT  24. 
Study  of  the  Transformer. 

See  pages  268,  269,  270,  271  and  Articles  161,  164  and  165  in 
the  text  book. 

The  object  of  this  experiment  is  a  study  of  the  principles  of 
construction  and  operation  of  the  transformer. 

Theory. — If  two  electrically  separate  coils  of  wire  be  wound  on 
the  same  iron  core,  one  of  which  is  connected  to  alternating  cur- 
rent supply  mains,  the  rapid  reversals  of  magnetism  produced 
by  the  alternating  current  in  the  one  coil  (the  primary)  set  up 


84. 


LABORATORY  MANUAL 


an  induced  alternating  electromotive  force  in  the  other  coil  (the 
secondary) . 

The  reversals  of  magnetism  induce  an  electromotive  force  in 
the  primary  as  well  as  in  the  secondary,  and  this  induced  elec- 
tromotive force  (sometimes  called  counter  electromotive  force) 
opposes  the  impressed  electromotive  force,  thus  keeping  the  pri- 
mary current  down  to  a  low  value  when  no  current  is  being  de- 
livered by  the  secondary  coil.  If  the  second  coil  delivers  current 
to  lamps,  the  magnetic  field  produced  by  it  passes  through  the 
primary  coil  and  the  counter  electromotive  force  of  the  primary 
coil  is  reduced,  thus  permitting  the  necessary  increase  in  primary 
current  to  maintain  the  load  on  the  secondary.  Note  that  the 


No. 

Primary 

Secondary 

No   of  Coils 

Series  or  Parallel 

Ei 

Ii 

No.  of  Coils 

Series  or  Parallel 

E2 

Ij 

1 

2 

3 

Form  14. 

primary  current,  with  no  load  on  the  secondary,  is  merely  that 
required  to  magnetize  the  iron  core. 

Since  the  magnetic  field  in  the  primary  coil  is  roughly  pro- 
portional to  the  number  of  turns  of  wire  for  a  given  current, 
and  since  the  electromotive  force  induced  in  the  secondary  coil 
by  the  given  rapidly  changing  magnetic  field  is  proportional  to 
the  number  of  turns  of  wire  in  the  coil,  the  induced  electromotive 
force  in  the  secondary  winding  (E2)  is  related  to  the  impressed 
electromotive  force  (E^  in  the  primary  directly  as  the  number 
of  turns  of  wire  in  the  two  coils.  Obviously  the  power  delivered 
by  the  secondary  winding  (E2I2)  is  equal  to  the  power  received 
by  the  primary  winding  (EJ.J  (ignoring  the  slight  losses  in  the 
transformer),  hence,  the  current  in  the  secondary  (72)  is  related 
to  the  current  in  the  primary  (7J  inversely  as  the  number  of 
turns  of  wire  in  the  two  coils. 


ALTERNATING  CURRENT  85 

Current  Supply. — 110  volts  Alternating  Current. 

Apparatus  Required. —  (1)  An  iron  core  with  a  permanently 
wound  fine  wire  coil  in  four  equal  parts  with  taps  to  be  used  as 
a  primary,  and  four  rather  coarse  insulated  wire  coils,  to  be  used 
as  a  secondary,  wound  with  the  same  number  of  turns  each  as 
the  primary  coils,  and  with  taps;  (2)  an  uninsulated  copper 
ring  to  fit  loosely  over  a  reactance  coil  with  an  iron  core ;  ( 3 )  two 
ammeters;  and  (4)  voltmeter. 

Order  of  Work. — 1.  Using  one  of  the  heavy  wire  coils,  connect 
one  of  the  primary  coils  to  the  supply  mains,  and  observe  and  re- 
cord the  number  of  turns  in  each  coil,  the  primary  volts  (E^, 
and  secondary  volts  (E2).  Use  Form  14. 

2.  Same,  with  2,  3  and  4  primary  coils  in  series,  in  turn. 

3.  With  the  four  primary  coils  in  series,  use  two  of  the  sec- 
ondary coils  in  series  and  repeat  the  observations  called  for  in 
item  1. 

4.  Same,  for  3  and  4  secondary  coils  in  series,  in  turn. 

5.  With  the  four  primary  coils  in  series  and  one  of  the  sec- 
ondary coils,  connect  the  secondary  terminals  to  a  lamp  bank  and 
observe  and  record  Elf  E2, 1^  and  72. 

6.  Same,  using  2,  3,  and  4  secondary  coils  in  series  and  2  and 
4  secondary  coils  in  parallel,  in  turn. 

7.  With  the  reactance  coil  connected  to  the  supply  mains,  place 
the  copper  ring  over  the  iron  core,  holding  it  with  a  pair  of  pliers. 

8.  Inspect  the  windings,  the  iron  core  and  the  case  of  a  com- 
mercial transformer,  making  a  rough  sketch  with  dimensions. 

Written  Report. — 1.  Calculate  the  relation  of  primary  to  sec- 
ondary turns,  and  E^  to  E2  in  item  1,  Order  of  Work.  Explain. 

2.  Same,  for  items  2,  3  and  4,  Order  of  Work.    Explain. 

3.  Calculate  the  relation  of  primary  to  secondary  turns,  E^  to 
E2,  and  I±  to  72  in  items  5  and  6,  Order  of  Work.    Explain. 

4.  Explain  the  phenomena  observed  in  item  7,  Order  of  Work. 

5.  In  a  step-down  transformer  (El  high  and  E2  low)  why  is 
the  primary  coil  wound  with  fine  wire,  and  the  secondary  with 
heavy  wire  ? 

6.  Why  is  the  iron  core  of  a  transformer  laminated  ? 

7.  Has  the  nature  of  the  space  between  the  coils  and  the  outer 
case,  and  the  construction  and  size  of  the  outer  case  anything  to 
do  with  the  power  rating  of  a  transformer  ?    Explain. 


86  LABORATORY  MANUAL 

EXPERIMENT  25. 

Electrical  Features  of  the  Transformer  and  the  Transmission  of 

Power. 

See  Article  69  in  the  text  book,  also  the  Theory  under  Experi- 
ment 3  in  the  Manual. 

The  object  of  this  experiment  is  (a)  to  gain  facility  in  the 
handling  of  a  commercial  transformer;  and  (b)  to  study  the 
effect  of  the  voltage  value  on  the  transmission  of  power. 

Theory. — The  coils  of  a  commercial  transformer  usually  end  in 
a  terminal  board  which  is  accompanied  by  a  diagram  of  connec- 
tions for  the  various  available  voltage  combinations.  While  these 
connections  are  ordinarily  shown  on  this  diagram,  it  is  advisable 
to  know  that  in  connecting  the  two  secondary  coils  in  series,  the 
two  coil  terminals  connected  together  must  be  of  opposite  po- 
larity at  the  same  instant ;  also  in  connecting  the  two  secondary 
coils  in  parallel,  the  terminal  of  one  coil  must  be  connected  to  the 
terminal  of  the  second  coil  possessing  the  same  polarity  at  each 
instant.  (This  also  applies  to  the  two  primary  coils.) 

In  the  transmission  of  a  given  amount  of  power  (El)  over  long 
distances,  the  volts  (RI)  drop  and  the  watts  (RP)  loss  in  the 
transmission  lines  obviously  depend  on  the  resistance  of  the  lines 
and  on  the  current.  Hence,  for  a  given  size  and  weight  of  wire 
(that  is,  for  a  given  resistance  of  the  lines)  the  losses  of  trans- 
mission are  less  the  smaller  the  current.  Since  El  represents  the 
power,  the  transmission  losses  are,  therefore,  reduced  for  a  given 
amount  of  power  transmitted,  by  using  a  high  voltage  (E)  and  a 
small  current  (I).  The  use  of  high  voltages  for  transmission  is 
made  possible  by  the  transformer,  which  is  arranged  to  step-up 
the  voltage  of  the  generator  at  the  beginning  of  a  long  line,  and  a 
second  transformer  at  the  end  of  the  line  steps  down  the  voltage 
to  a  practical  working  value,  thus  securing  all  the  advantages  of 
high  voltage  (that  is  low  current)  throughout  the  length  of  the 
transmission  wires. 

Current  Supply. — 110  volts  Alternating  Current. 

Apparatus  Required. —  (1)  Two  commercial  transformers  with 
four  coils  each  (that  is,  two  primary  and  two  secondary  coils 
each),  and  preferably  with  a  one  to  one  ratio;  (2)  lamp  bank 


ALTERNATING  CURRENT 


87 


to  be  used  as  a  receiving  circuit;  (3)  three  ammeters;  and  (4) 
voltmeter. 

Order  of  Work. — 1.  Connect  the  primary  of  one  of  the  trans- 
formers to  the  supply  mains  (with  the  two  primary  and  the  two 
secondary  coils  each  in  parallel),  and  connect  the  secondary  to 
the  line.  Arrange  the  second  transformer  in  the  same  way  as  re- 
gards the  connection  of  its  coils,  attaching  the  terminals  at  the 
end  of  the  line  to  its  primary  and  the  lamps  to  its  secondary  as 
shown  in  Fig.  25. 


Supply  Mains   (110  Volts  60  Cycles  A.  C.) 


Supply  Transformer 
Usually  Step  Up  Transformation) 


Lamp  Bank  Used  as  Load 


Fig.  25. — Study  of  the  transformer  and  the  transmission  of  power  at 
different  voltages.  The  short  transmission  line  should  be  of  such 
length  and  size  as  to  give  a  voltage  and  line  loss  equal  to,  say,  ten  per 
cent,  of  the  power  transmitted  in  the  first  case. 


2.  Observe  and  record  Elf  E2,  7X  and  72  in  each  of  the  two 
transformers. 

3.  Connect  the  two  transformers  so  that  the  line  voltage  is 
double  the  value  used  in  items  1  and  2,  recording  a  diagram  of 
the  transformer  coil  connections,  and  with  the  same  load  on  the 
secondary  of  the  receiving  transformer,  observe  and  record  Elf 
E2t  II  and  I2  in  each  transformer. 


88  LABORATORY  MANUAL 

Written  Report. — 1.  Calculate  the  voltage  and  power  loss  in  the 
two  lines  from  the  observations  in  items  1  and  2,  Order  of  Work. 

2.  Same,  for  item  3,  Order  of  Work. 

3.  How  do  the  voltage  and  power  losses  compare  with  the  two 
different  line  voltages  ?    Explain. 

4.  Explain,    using   diagrams,    what    transformer    connections 
were  used  to  secure  the  double  line  voltage  in  item  3,  Order  of 
Work. 

5.  Why  must  the  two  terminals,  connected  together  in  the  case 
of  a  two-coil  secondary,  be  opposite  in  polarity  at  each  instant 
for  the  series  connection  of  the  two  coils  ?    Explain.    What  would 
result  if  these  two  terminals  were  of  the  same  polarity  at  each 
instant  ? 

6.  Why  must  the  terminal  of  one  coil  be  connected  to  a  ter- 
minal of  the  second  coil  having  the  same  polarity  at  each  instant 
in  a  two-coil  secondary  for  parallel  connection  of  the  two  coils? 
Explain.    What  would  result  if  these  two  terminals  were  of  oppo- 
site polarity  at  each  instant? 

7.  From  the  observations  in  this  experiment,  explain  why  high 
voltage  increases  the  efficiency  of  transmission  of  electric  power 
over  long  distances. 


EXPERIMENT  26. 
Study  of  the  Induction  Motor. 
See  Fig.  385a,  page  497  in  the  text  book. 

The  object  of  this  experiment  is  to  gain  a  working  knowledge 
of  the  construction,  and  to  observe  the  principles  involved  in 
the  production  of  motion  in  the  induction  motor. 

Theory. — If  the  field  poles  of  a  direct  current  motor  were  ro- 
tated about  the  armature,  induced  electromotive  forces  would  be 
set  up  in  the  armature  conductors.  If,  further,  the  armature 
conductors  were  so  connected  that  the  induced  electromotive 
forces  thus  set  up  produced  a  flow  of  current,  the  armature  wires 
carrying  current  would  be  acted  on  by  the  moving  magnetic  field 
thus  producing  a  mechanical  force  or  torque  tending  to  cause 
the  armature  to  rotate. 

In  the  induction  motor  a  rotating  magnetic  field  is  produced 
by  alternating  current  supplied  to  the  stationary  field  windings 


ALTERNATING  CURRENT  89 

(called  the  stator)  and  this  rotating  field  acts  on  short  circuited 
conductors  thus  inducing  currents.  This,  obviously,  fulfills  the 
condition  for  motor  action,  namely,  wires  carrying  current  in  a 
magnetic  field.  It  will  be  noted  that,  while  the  magnetism  is  ro- 
tating, the  field  windings  are  themselves  stationary,  the  rotating 
field  effect  being  made  possible  by  the  rapidly  changing  and  re- 
versing alternating  current  in  the  stator.  The  production  of  this 
rotating  magnetic  field  by  the  alternating  current  is  one  of  the 
objects  of  study  in  this  experiment. 

Current  Supply. — 110  volts  Direct  Current,  a  low  frequency 
Alternating  Current  and  Alternating  Current  of  the  same  fre- 
quency as  the  motor  assigned. 

Apparatus  Required. —  (1)  The  field  (stator)  of  a  three-phase 
induction  motor  with  the  rotor  removed;  and  (2)  a  simple  iron 
device  corresponding  to  a  compass  needle,  pivoted  in  the  position 
of  the  rotor  to  indicate  the  poles  of  the  stator;  (3)  brake  to  be 
used  for  loading  the  motor;  (4)  device  for  measuring  the  slip; 
(5)  voltmeter;  and  (6)  ammeter.  (If  a  two-phase  induction 
motor  is  used,  the  same  instructions  apply  with  obvious  modifica- 
tions.) 

Order  of  Work. — 1.  Mark  or  label  the  three  terminals  of  the 
stator  winding  as  "  1,"  "2"  and  "3,"  in  consecutive  order,  and 
connect  the  direct  current  supply  mains  through  a  suitable  pro- 
tective resistance  to  the  terminals  1  and  2,  then  to  2  and  3,  then 
to  3  and  1  in  turn,  going  through  this  succession  several  times. 
Make  a  sketch  showing  the  succcessive  positions  of  the  pointer  as 
indicating  the  magnetic  field  rotation.  Observe  the  number  of 
poles  of  the  stator. 

2.  Connect  the  stator  to  a  low  frequency  supply  and  observe 
the  action  on  the  pivoted  pointer. 

3.  Make  a  sketch  of  the  arrangement  of  several  coils  in  the 
stator  winding  as  related  to  each  other,  that  is,  showing  the  an- 
gular displacement  of  the  various  coils. 

4.  Make  a  diagram  of  the  rotor  winding  showing  the  details  of 
the  end  connections  of  the  rotor  conductors. 

5.  Place  the  rotor  in  position  in  its  bearings  and  connect  the 
stator  to  the  rated  supply  mains  (alternating  current)  allowing 
the  machine  to  come  up  to  its  normal  speed.     Observe  and  re- 


90  LABORATORY  MANUAL 

cord  the  speed  in  revolutions  per  minute  and  note  the  frequency 
of  the  circuit  used. 

6.  With  normal  voltage  across  the  motor  terminals,  load  by 
means  of  a  brake,  and  observe  and  record  the  torque,  current, 
and  slip  with  increasing  load,  that  is,  for  zero,  14,  %,  %  and 
full  load  in  turn. 

7.  Same  as  item  6,  except  that  %  of  the  normal  voltage  value 
is  to  be  used. 

Written  Report. — 1.  Explain  just  how  a  three-phase  current 
produces  a  rotating  field  effect  similar  to  that  produced  in  item 
1,  Order  of  Work. 

2.  What  determines  the  number  of  poles  in  the  stator  where 
the  windings  are  flush  with  the  iron  ? 

3.  What  is  the  object  in  short  circuiting  the  rotor  conductors? 

4.  Prom  the  observation  of  the  number  of  poles  in  1  and  the 
frequency  in  item  5,  Order  of  Work,  calculate  the  number  of 
revolutions  per  minute  of  the  rotating  field. 

5.  What  is  the  numerical  difference  between  the  number  of 
revolutions  per  minute  of  the  rotor  as  observed  in  item  5,  Order 
of  Work,  and  that  of  the  stator  as  calculated  in  item  4,  Written 
Report?     (Note  that  the  difference  between  these  speeds  is  the 
slip  of  the  motor. ) 

6.  Plot  two  curves  on  the  same  sheet,  using  the  speed  of  the 
rotor   (found  from  the  observations  of  stator  magnetism  speed 
and  the  slip)  as  abscissas  and  torque  as  ordinates,  from  items  6 
and  7,  Order  of  Work. 


EXPERIMENT  27. 

Electrical  Features  of  the  Induction  Motor. 
See  the  Theory  under  Experiment  26  in  the  Manual. 

The  object  of  this  experiment  is  to  observe  the  conditions  which 
affect  the  speed  and  torque  of  the  induction  motor. 

Theory. — The  induction  motor  may  roughly  be  considered  a 
constant  speed  machine  under  constant  load  conditions.  How- 
ever, as  the  load  increases,  the  rotor  speed  falls  and  the  difference 
between  the  speed  of  the  rotating  field  and  that  of  the  rotor,  name- 


ALTERNATING  CURRENT  91 

ly  the  slip  (see  item  5  under  the  heading,  Written  Report,  in  Ex- 
periment 26)  increases. 

Note  that  the  magnetic  field  rotates  at  a  speed  which  is  pro- 
portional to  the  frequency  of  the  supply  current.  Thus,  if  the 
frequency  be  reduced  to  half  its  original  value  the  magnetic  field 
rotates  at  half  the  original  speed.  The  rotor  speed  depends  at  no- 
load  on  this  rotational  speed  of  the  magnetic  field  and,  hence,  for 
half  frequency  the  rotor  speed  (at  no-load)  is  reduced  to  about 
half  value. 

On  the  other  hand,  the  torque  of  the  motor  depends  on  the 
magnetic  field  (which  is  proportional  to  the  impressed  voltage 
E)  and  on  the  rotor  current  (proportional  to  the  magnetic  field 
and,  hence,  to  the  impressed  voltage).  The  torque  may,  there- 
fore, be  said  to  depend  on  the  square  of  the  impressed  voltage 
(E2). 

Hence,  if  the  frequency  be  maintained  constant  and  the  im- 
pressed voltage  reduced  to  half  value,  the  no-load  speed  of  the 
motor  will  remain  sensibly  the  same  as  before  (since  the  magnetic 
field  rotates  as  before  and  the  torque  required  at  no-load  is  negli- 
gible). If,  however,  when  the  motor  is  loaded,  the  impressed 
voltage  be  reduced  to  half  value,  the  frequency  being  maintained 
constant,  the  slip  will  be  increased  because  the  available  torque 
is  reduced  by  the  reduction  of  E  and  this  must  be  made  up  by  the 
larger  rotor  currents  set  up  in  this  case  by  the  higher  induced 
electromotive  force  in  the  rotor  due  to  the  greater  relative  mo- 
tion between  rotor  and  field  or,  in  other  words,  to  the  greater 
slip. 

Note,  also,  that  the  frequency  of  the  rotor  currents  is  highest 
when  the  rotor  is  at  rest,  that  is,  at  starting,  hence,  the  react- 
ance of  the  rotor  winding  is  greatest  at  this  time,  and  the  power 
factor  of  the  rotor  at  its  lowest.  This  means  that  the  magnetic 
field  and  the  rotor  currents  (the  two  factors  which  produce  the 
motion)  are  at  their  maximum  phase  difference  when  the  motor 
is  starting.  Since  the  most  advantageous  condition  for  the  torque 
is  when  the  field  and  rotor  currents  are  in  phase  with  each  other 
(or  at  their  least  phase  difference)  it  is  obvious  that  the  torque 
at  starting  is  lower  than  at  any  other  point  of  the  operation  of 
the  motor  due  to  the  low  power  factor. 

The  principal  means  for  improving  the  power  factor  at  start- 
ing and  thus  improving  the  starting  torque,  is  to  insert  an  auxili- 


92  LABORATORY  MANUAL 

ary  resistance  in  series  with  the  rotor,  which  is  all  cut  in  at  start- 
ing and  gradually  cut  out  as  the  rotor  comes  up  to  normal  speed. 

Current  Supply. — Three-phase  Alternating  Current. 

Apparatus  Required. —  (1)  Three-phase  induction  motor  with 
auxiliary  resistance  arranged  in  the  rotor  circuit;  (2)  brake  to 
be  used  for  loading  the  motor ;  (3)  device  for  measuring  the  slip  ; 
(4)  ammeter;  and  (5)  voltmeter. 

Order  of  Work. — 1.  Connect  the  ammeter  in  one  of  the  leads 
of  the  stator  circuit  and  arrange  to  drive  the  motor  from  supply 
mains  with  its  normal  voltage  and  frequency. 

2.  Bun  the  motor  unloaded,  and  observe  and  record  the  slip 
for  normal  and  for  %  voltage. 

3.  Starting  at  zero  load  with  all  the  auxiliary  resistance  cut 
out,  and  at  %,  %,  %,  and  full  load  in  turn,  observe  and  record 
the  slip  and  the  torque  for  normal  voltage  throughout. 

4.  Same  as  item  3,  for  %  voltage,  as  far  as  the  load  can  be  car- 
ried. 

5.  Same  as  item  37  with  %  the  auxiliary  resistance  cut  in. 

6.  Same  as  item  3,  with  all  the  auxiliary  resistance  cut  in. 

7.  Tighten  the  brake,  and  measure  the  current  in  one  line  and 
the  torque  at  the  pulley  at  starting,  with  zero  auxiliary  resistance 
in  the  rotor  circuit  and  using  %  voltage  to  prevent  the  flow  of  an 
excessive  current. 

8.  Same  as  item  7,  with  %  the  auxiliary  resistance  in  the  ro- 
tor circuit  and  using  %  voltage. 

9.  Same  as  item  7,  using  all  the  auxiliary  resistance  in  the  ro- 
tor circuit  and  %  voltage. 

10.  With  all  the  auxiliary  resistance  in  the  rotor  circuit,  re- 
peat the  observations  called  for  in  item  7,  first,  at  %  voltage,  and 
second,  at  normal  voltage. 

Written  Report. — 1.  How  does  the  slip  compare  for  the  two 
voltages  in  item  2,  Order  of  Work?  Explain. 

2.  Plot  four  curves  on  the  same  sheet  from  the  observations 
of  items  3,  4,  5,  and  6,  Order  of  Work,  and  explain  from  these 
curves  how  the  speed  and  torque  vary  with  the  amount  of  auxili- 
ary resistance  in  the  rotor  circuit. 

3.  What  effect  on  the  speed  and  torque  is  produced  in  item  4, 
Order  of  Work,  by  the  reduced  voltage?    Explain. 


ALTERNATING  CURRENT  93 

4.  Compare  the  starting  torque  for  items  7,  8  and  9.     How 
does  the  auxiliary  resistance  in  the  rotor  circuit  affect  the  start- 
ing torque  ?    Explain. 

5.  How  does  the  slip  at  normal,  and  at  %  voltage,  in  item  2, 
Order  of  Work,  compare  with  the  corresponding  values  in  items 
3  and  4,  Order  of  Work,  at  full  load?    Explain. 

6.  In  item  10,  how  does  the  starting  torque  compare  at  */>,  and 
at  normal  voltage  ?    Explain. 


EXPERIMENT  28. 
Study  of  the  Synchronous  Motor. 
See  Fig.  411,  page  515  in  the  text  book. 

The  object  of  this  experient  is  to  secure  an  idea  as  to  the  gen- 
eral operation  of  the  alternator  when  run  as  a  motor  (usually  re- 
ferred to  as  the  synchronous  motor) . 

Theory. — In  an  alternating  current  generator  (commonly 
called  an  alternator)  the  magnetic  field  is  produced  by  direct 
current  from  a  separate  source  of  supply,  while  the  current  in 
the  armature  conductors  is,  of  course,  alternating  current.  If 
the  alternator  is  used  as  a  motor,  the  field  winding  is  supplied 
by  direct  current  as  before,  and,  as  alternating  current  is  sup- 
plied to  the  armature  conductors,  it  will  be  apparent,  after  a 
little  study,  that  the  current  in  a  given  conductor  must  reverse 
in  direction  in  exactly  the  time  taken  for  the  conductor  to  pass 
from  one  pole  to  an  adjacent  pole  in  order  that  the  mechanical 
force  on  each  conductor  may  always  be  in  the  same  direction  as 
the  motion. 

In  other  words,  the  motor  must  run  at  the  same  frequency  as 
the  alternator  which  supplies  it  with  power,  and  for  this  reason 
the  motor  is  called  the  synchronous  motor.  When  the  synchro- 
nous motor  is  started,  it  must,  in  general,  be  brought  up  to  syn- 
chronous speed  by  some  outside  mechanical  means  (an  auxiliary 
motor),  that  is,  to  the  same  frequency  as  the  alternator,  and  the 
motor  must  further  be  connected  to  the  supply  mains  at  that  in- 
stant when  the  electromotive  force  of  the  armature  (called 
counter  electromotive  force  after  the  machine  is  running  as  a 
motor)  is  equal  to  that  of  the  alternator.  This  operation  is  termed 
synchronizing. 


94 


LABORATORY  MANUAL 


After  being  synchronized,  the  motor  continues  to  run  at  syn- 
chronous (same  frequency)  speed  at  all  loads,  unless,  due  to  an 
overload  its  speed  be  momentarily  brought  lower  than  synchro- 
nous speed,  in  which  case  it  stops  and  must  be  started  as  before. 
Note  that  the  synchronous  motor  is  a  constant  speed  motor  and 
that  its  speed  cannot  be  adjusted. 

Very  often  the  synchronous  motor  forms  a  part  of  what  is 
termed  a  synchronous  converter  (or  rotary  converter),  namely,  a 


Supply  Mains 

(110  Volts  60  Cycles  A.  C.) 

JL 

Synchronous  Motor 

Fig.  26. — Synchronous  motor.  Note  that  the  main  switch  is  to  be 
thrown  in  before  the  synchronizing  switch,  and  the  latter  not  until  the 
conditions  for  synchronizing  are  fulfilled.  The  lamp  should  be  220 
volts,  or  two  110-volt  lamps  may  be  used  in  series. 


machine  arranged  with  collector  rings  at  one  end  and  a  commu- 
tator at  the  other  end  of  the  armature,  each  connected  to  the 
same  armature  winding.  The  common  use  of  the  rotary  con- 
verter is  to  operate  it  as  a  synchronous  motor  from  the  collector 
ring  side,  for  example,  in  a  railway  sub-station,  and  to  supply 
direct  current  to  the  railway  line  from  the  commutator  end  of 
the  machine.  The  rotary  converter  can,  however,  be  used  to  run 
as  a  direct  current  motor  and  supply  alternating  current,  in 
which  case  it  is  sometimes  referred  to  as  an  inverted  rotary  con- 
verter. 


ALTERNATING  CURRENT  95 

Current  Supply. — 110  or  220  volts  Alternating  Current,  and 
110  or  220  volts  Direct  Current  (for  field  excitation). 

Apparatus  Required. —  (1)  An  alternator  to  be  run  as  a  motor; 
(2)  field  rheostat;  (3)  incandescent  lamp  to  be  used  for  syn- 
chronizing; (4)  speed  indicator;  (5)  two  ammeters;  (6)  watt- 
meter; and  (7)  voltmeter. 

Order  of  Work. — 1.  Connect  the  alternator  for  synchronous 
motor  operation  as  shown  in  Fig.  26. 

2.  Bring  up  the  motor  to  speed  by  an  auxiliary  motor  (or  by 
direct  current  if  a  rotary  converter  is  used),  and  adjust  the  volt- 
age to  the  value  of  the  supply  mains.    Determine,  by  the  lamp, 
when  the  conditions  for  synchronizing  are  realized  and  connect 
the  motor  to  the  line.    Disconnect  the  driving  motor  (or  the  di- 
rect current  supply  to  the  rotary  converter  as  the  case  may  be). 

3.  Measure  and  record  the  speed  of  the  motor,  and  observe  the 
number  of  poles  and  the  frequency  of  the  supply  alternator. 

4.  "With  the  synchronous  motor  in  operation  (unloaded)  keep 
the  applied  electromotive  force  constant,  and  measure  and  record 
the  armature  volts  and  amperes,  field  amperes  and  watts  deliv- 
ered to  the  armature  with  various  field  currents,  above  and  be- 
low that  value  giving  minimum  armature  current. 

Written  Report. — 1.  Explain  in  detail  the  necessary  conditions 
for  synchronizing. 

2.  Compare  the  speed  of  the  alternator  as  calculated  from  the 
observations  in  item  3,  Order  of  Work,  with  the  observed  speed  of 
the  motor. 

3.  Just  why  must  the  current  in  a  given  synchronous  motor 
conductor  reverse  each  time  the  conductor  passes  from  a  north  to 
an  adjacent  south  pole? 

4.  In  a  direct  current  motor,  the  field  is  of  course  furnished  by 
direct  current  and  although  the  machine,  as  a  whole,  is  supplied 
with  direct  current,  the  commutator  causes  the  current  in  the 
armature  conductors  to  be  alternating.     Explain  just  how  the 
mechanical  force  in  the  direct  current  shunt  motor  armature  con- 
ductors is  always  automatically  in  the  direction  of  motion. 

5.  For  each  of  the  sets  of  observations  in  item  4,  Order  of 
Work,  calculate  the  apparent  watts  (El)  input  to  the  armature 
and  the  power  factor  (true  watts  as  indicated  by  the  wattmeter 


96  LABORATORY  MANUAL 

divided  by  the  apparent  watts).    Explain  any  variations  in  the 
power  factor  for  the  different  values  of  field  current. 

6.  Why  is  the  armature  current  greater  when  the  field  current 
is  above  or  below  that  value  which  gives  minimum  armature  cur- 
rent ? 


EXPERIMENT  29. 
Alternators  in  Parallel. 

See  the  Theory  under  Experiments  19,  20  and  28  in  the  Man- 
ual. 

Theory. — The  conditions  to  be  met  in  the  operation  of  direct 
current  generators  in  parallel  must  also  be  met  in  the  parallel 
operation  of  alternators,  namely,  the  polarity  of  the  terminals 
connected  to  a  given  bus  bar  and  the  voltage  must  be  the  same, 
and,  further,  the  two  machines  must  have  the  same  frequency. 

This  means  that  if  one  alternator  is  connected  to  the  bus  bars, 
and  a  second  one  is  to  be  connected  in  parallel  with  the  first,  the 
second  machine  must  be  synchronized  with  the  first,  like  the  case 
described  in  Experiment  28.  Further,  if  the  two  machines  have 
the  same  number  of  poles,  they  must  obviously  run  at  the  same 
speed  to  have  the  same  frequency. 

The  conditions  to  be  met  in  throwing  one  alternator  in  parallel 
with  another  may  be  summarized  as  follows : 

(a)  Each  machine  must  have  the  same  terminal  voltage. 

(b)  The  two  terminals  to  be  connected  to  the  same  bus  bar 
must  be  of  the  same  polarity  at  each  instant,  namely,  they  must 
be  of  the  same  phase. 

(c)  The  machines  must  have  the  same  frequency. 

Current  Supply. — 110  or  220  volts  Direct  Current  (for  field  ex- 
citation), and  the  Alternating  Current  from  the  alternators  as- 
signed. 

Apparatus  Required. —  (1)  Two  alternators;  (2)  lamp  for  syn- 
chronizing; (3)  speed  indicator;  (4)  lamp  bank  to  be  used  as  a 
load  for  the  bus  bars;  (5)  three  ammeters,  one  for  each  machine 
and  one  for  the  total  bus  bar  load;  and  (6)  two  voltmeters. 


ALTERNATING  CURRENT 


97 


Order  of  Work. — 1.  Arrange  the  machines  and  instruments  as 
shown  in  Fig.  27.  Starting  up  the  machines,  bring  them  to  nor- 
mal speed  and  voltage,  and  connect  one  machine  to  the  bus  bars 
(two  lengths  of  wire).  Synchronize  the  second  machine  with  the 
first  and  connect  it  to  the  bus  bars  at  the  proper  instant  as  indi- 
cated by  the  lamp  (used  for  synchronizing). 


Alternator  "1 


Alternator  "2" 


Synchronizing  Switch 


Fig.  27. — Parallel  operation  of  alternators.  A  voltmeter,  not  shown 
in  the  diagram,  is  to  be  available  for  measuring  the  voltage  of  the  in- 
dividual machines. 


2.  Load  the  machines  from  the  bus  bars  until  each  delivers  its 
full  rated  load  (or  some  fraction  of  full  load),  making  needed  ad- 
justments of  load  between  the  machines  with  the  field  rheostats. 
Observe  and  record  the  current  delivered  by  each  machine  and  by 
the  bus  bars. 

3.  Same,  for  %  the  value  of  current  used  in  item  2,  leaving 
the  field  rheostats  untouched  in  the  positions  as  in  item  2. 

8 


98  LABORATORY  MANUAL 

4.  With  each  machine  delivering  about  ^2  its  rated  load  to  the 
bus  bars,  throw  all  the  load  to  one  of  the  machines  and  discon- 
nect the  unloaded  machine  from  the  bus  bars. 

5.  Start  the  machines,  and  load  the  bus  bars  until  each  ma- 
chine is  delivering,  say,  full  load  (or  some  fraction  of  full  load). 
Raise  the  field  current  in  one  of  the  machines  and  lower  it  in 
the  other,  keeping  the  bus  bar  voltage  constant.    Observe  and  re- 
cord the  effect  on  (a)  the  armature  currents  of  the  two  machines 
and  their  arithmetical  sum;  (b)  the  total  load  current  from  the 
bus  bars;  (c)  power  output  (El  in  the  case  of  lamps)  ;  and  (d) 
power  delivered  by  each  machine. 

Written  Report. — 1.  Explain  briefly  just  why  the  three  condi- 
tions for  throwing  two  generators  in  parallel  must  be  met,  as  de- 
scribed under  the  Theory. 

2.  Why  are  these  conditions  fulfilled  when  the  synchronizing 
lamp  is  dark  (or  light,  depending  on  which  method  was  used)  ? 

3.  Do  the  machines  share  the  total  bus  bar  load  equally  when 
the  load  is  reduced  to  half  value  in  item  3,  Order  of  Work?    Ex- 
plain. 

4.  Why  is  it  necessary  to  reduce  the  load  on  the  one  machine 
to  zero  before  disconnecting  it  from  the  bus  bars  ? 

5.  What  would  be  the  effect  if  one  of  the  machines  were  discon- 
nected from  the  bus  bars  while  delivering  its  share  of  the  total 
bus  bar  load  ? 

6.  From  the  observations  obtained  in  item  5,  Order  of  Work, 
state  and  explain  the  effect  produced  on  the  armature  currents 
of  the  two  machines  and  their  arithmetical  sum ;  the  total  load 
current  from  the  bus  bars;  the  power  output  (total)  ;  and  the 
power  output  from  each  machine,  as  the  field  currents  are  ad- 
justed. 

EXPERIMENT  30. 
Study  of  the  Mercury  Arc  Rectifier. 

The  object  of  this  experiment  is  to  afford  the  opportunity  for 
observing  the  construction  and  operation  of  the  Mercury  Arc 
Rectifier. 

Theory. — The  function  of  this  device  is  to  make  a  uni-direc- 
tional  (one  direction)  current  from  alternating  current.  Its 


ALTERNATING  CURRENT  99 

principal  uses  are  in  connection  with  arc  lighting  systems  where 
direct  current  arc  lamps  of  the  Magnetite  or  Metallic  Flame  type 
are  used  (not  operative  on  alternating  current  circuits)  and 
where  the  advantages  of  distribution  of  the  power  by  alternating 
current  make  it  an  economy  to  install  this  auxiliary  piece  of  ap- 
paratus for  transforming  the  alternating  to  direct  current ;  also 
where  storage  batteries  are  to  be  charged  (with  direct  current) 
and  alternating  current  is  the  only  available  supply. 

The  principle  of  operation  depends  on  the  fact  that  mercury 
placed  in  a  vacuum  bulb  with  one  terminal  in  contact  with  the 
mercury  and  one  terminal  above  the  line  of  the  mercury,  permits 
current  to  flow  through  it  in  one  direction  only.  An  ingenious 
device  permits  both  alternations  in  each  cycle  to  be  used  in  con- 
nection with  the  rectifier. 

Current  Supply. — 110  or  220  volts  Alternating  Current. 

Apparatus  Required. —  (1)  Mercury  arc  rectifier;  (2)  lamp 
bank  to  be  used  as  a  load ;  (3)  two  ammeters;  (4)  two  voltmeters ; 
and  (5)  a  wattmeter. 

Order  of  Work. — 1.  Connect  the  rectifier  switch  to  the  alternat- 
ing current  supply  mains  and  arrange  to  load  the  outfit  with  the 
lamps,  inserting  an  ammeter  and  voltmeter  on  each  side  of  the 
rectifier,  and  a  wattmeter  on  the  alternating  current  side. 

2.  Start  up  the  rectifier  and  connect  in  the  alternating  cur- 
rent supply.     Turn  on  lamps  up  to  the  capacity  of  the  rectifier, 
and  observe  and  record  the  current  and  voltage  on  each  side  of 
the  rectifier  and  the  watts  on  the  alternating  current  side.    Note 
carefully  the  tilting  device  for  starting  the  rectifier  action. 

3.  Same,  for  %,  %  and  %  loads,  in  turn. 

Written  Report. — 1.  Describe  briefly  the  action  of  the  Mercury 
Arc  Rectifier. 

2.  From  the  observations  in  items  2  and  3,  Order  of  Work,  cal- 
culate the  efficiency  of  the  rectifier  at  full  load  and  at  %,  %  and 
%  loads,  also  the  power  factor  for  each  load  on  the  alternating 
current  side  and  the  relation  of  the  alternating  voltage  to  the  di- 
rect current  voltage  in  each  case. 


100  LABORATORY  MANUAL 

3.  What  regular  maintenance  item  must  be  considered  in  the 
operation  and  up-keep  of  the  rectifier  ? 

4.  Although  the  current  can  flow  in  one  direction  only  through 
the  rectifier  bulb,  the  statement  was  made  in  the  Theory  that 
both  alternations  in  each  cycle  are  utilized.    Explain  how  this  is 
possible. 


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ENG1NEEHINU  LI 


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